The burning of Tricky Tony, as it afterwards became known in the Circus mythology - such being Grigoriev's whimsical codename among the watchers - was one of those rare operations where luck, timing and preparation come together in a perfect marriage. They had all known from early on that the problem would be to find Grigoriev alone at a moment which allowed for his speedy reintroduction into normal life a few hours later. Yet by the weekend following the coverage of the Thun bank, intensive researches into Grigoriev's behaviour pattern had produced no obvious pointers as to when this moment might be. In desperation Skordeno and de Silsky, Toby's hard-men, dreamed up a wildcat scheme to snatch him on his way to work, along the few hundred metres of pavement between his house and the Embassy. Toby killed it at once. One of the girls offered herself as a decoy : perhaps she could hitch a lift from him somehow? Her altruism was applauded, but it did not answer the practicalities.

The main problem was that Grigoriev was under double guard. Not only did the Embassy security staff keep check on him as a matter of routine; so did his wife. The watchers had no doubt that she suspected him of a tenderness for little Natasha. Their fears were confirmed when Toby's listeners contrived to tamper with the junction box at the corner of the road. In one day's watch, Grigorieva telephoned her husband no less than three times, to no apparent purpose other than to establish that he was indeed at the Embassy.

'George, I mean that woman is a total monster,' Toby stormed when he heard this. 'Love - I mean, all right. But possession, for its own sake, this I absolutely condemn. It's a matter of principle for me.'

The one chink was Grigoriev's Thursday-afternoon drives to the garage, when he took the Mercedes to have it checked. If a practised car coper such as Canada Bill could introduce an engine fault during the Wednesday night - one that kept the car mobile, but only just - then might not Grigoriev be snatched from the garage while he was waiting for the mechanic to trace it? The plan bristled with imponderables. Even if everything worked, how long would they have Grigoriev to themselves? Then again, on Thursdays Grigoriev must be back home in time to receive his weekly visit from the courier Krassky. Nevertheless, it remained the only plan they had - their worst except for the others, said Toby - and accordingly they settled to an apprehensive wait of five days while Toby and his team leaden plotted fallbacks for the many unpleasant contingencies should the plot abore everyone to be signed out of his hotel and packed; escape papers and money to be carried at all times; radio equipment to be boxed and cached under American identity in the vaults of one of the major banks, so that any clues left behind would point to the Cousins rather than themselves; no forms of assembly other than walk-and-talk encounters on the pavement; wavelengths to be changed every four hours. Toby knew his Swiss police, he said. He had hunted here before. If the balloon went up, be said, then the fewer of his boys and girls around to answer questions, the better. 'I mean, thank God the Swiss are only neutral, know what I mean?'

As a somewhat forlorn consolation, and as a boost to the delicate morale of the watchers, Smiley and Toby decreed that the surveillance of Grigoriev should be kept at full pitch throughout the expected days of waiting. The observation post in the Brunnadernrain would be manned round the clock; car and cycle patrols would be increased; everyone should be on his toes for the remote chance that God, in an uncharacteristic moment, would favour the just.

What God did, in fact, was send idyllic Sunday weather, and it proved decisive. By ten o'clock that Sunday it was as if the Alpine sun had come down from the Oberland to brighten the lives of the fog-ridden lowlanders. In the Bellevue Palace, which on Sundays has a quite overwhelming calm, a waiter had just spread a napkin on Smiley's lap for him. He was drinking a leisurely coffee, trying to concentrate on the weekend edition of the Herald Tribune , when, looking up, be saw the gentle figure of Franz the head porter standing before him.

'Mr Barraclough, sir, the telephone, I am sorry. A Mr Anselm.'

The cabins were in the main hall, the voice was Toby's and the name Anselm signified urgency : 'The Geneva bureau has just advised us that the managing director is on his way to Berne at this very moment.'

The Geneva bureau was word code for the Brunnadernain observation post.

'Is he bringing his wife?' said Smiley.

'Unfortunately, Madame is obliged to make an excursion with the children,' Toby replied. 'Perhaps if you could come down to the office, Mr Barraclough?'

Toby's office was a sun pavilion situated in an ornamental garden next to the Bundeshaus. Smiley was there in five minutes. Below them lay the ravine of the green river. In the distance, under a blue sky, the peaks of the Bernese Oberland lifted splendidly in the sunlight.

'Grigoriev left the Embassy on his own five minutes ago, wearing a hat and coat,' Toby said as soon as Smiley arrived. 'He's heading for the town on foot. It's like the first Sunday we watched him. He walks to the Embassy, ten minutes later he sets off for the town. He's going to watch the chess game, George, no question. What do you say?'

'Who's with him?'

'Skordeno and de Silsky on foot, a back-up car behind, two more ahead. One team's heading for the Cathedral Qose right now. Do we go, George, or don't we?'

For a moment, Toby was aware of that disconnection which seemed to afflict Smiley whenever the operation gathered speed : less indecision, than a mysterious reluctance to advance.

He pressed him : 'The green light, George? Or not? George, please! We are speaking of seconds here!'

'Is the house still covered for when Grigorieva and the children get back?'

'Completely.'

For a moment longer Smiley hesitated. For a moment, he weighed the method against the prize, and the grey and distant figure of Karla seemed actually to admonish him.

'The green light, then,' said Smiley. 'Yes. Go.'

He had barely finished speaking before Toby was standing in the telephone kiosk not twenty metres from the pavilion. 'With my heart going like a complete steam engine,' as he later claimed. But also with the light of battle in his eyes.

There is even a scale model of the scene at Sarratt, and occasionally the directing staff will dig it out and tell the tale.

The old city of Berne is best described as a mountain, a fortress, and a peninsula all at once, as the model shows. Between the Kirchenfeldand Kornhaus bridges, the Aare runs in a horseshoe cut into a giddy cleft, and the old city roosts prudently inside it, in rising foothills of medieval streets, till it reaches the superb late-Gothic spire of the Cathedral, which is both the mountain's peak and its glory. Next to the Cathedral, at the same height, stands the Platform, from whose southern perimeter the unwary visitor may find himself staring down a hundred feet of sheer stone face, straight into the swirling river. It is a place to draw suicides and no doubt there have been some. It is a place where, according to popular history, a pious man was thrown from his horse and, though he fell the whole awesome distance, survived by God's deliverance to serve the church for another thirty years, dying peacefully at a great age. The rest of the Platform makes a tranquil spot, with benches and ornamental trees and a children's playground - and, in recent years, a place for public chess. The pieces are two foot or more in height, light enough to move, but heavy enough to withstand the occasional thrust of a south wind that whips off the surrounding hills. The scale model even runs to replicas of them.

By the time Toby Esterhase arrived there that Sunday morning, the unexpected sunshine had drawn a small but tidy body of the game's enthusiasts, who stood or sat around the chequered pavement. And at their centre, a mere six feet from where Toby stood, as oblivious to his surroundings as could be wished, stood Counsellor (Commercial) Anton Grigoriev of the Soviet Embassy in Berne, a truant from both work and family, intently following, through his rimless spectacles, each move the players made. And behind Grigoriev stood Skordeno and his companion de Silsky, watching Grigoriev. The players were young and bearded and volatile - if not art students, then certainly they wished to be taken for them. And they were very conscious of fighting a duel under the public gaze.

Toby had been this close to Grigoriev before, but never when the Russian's attention was so firmly locked elsewhere. With the calm of impending battle, Toby appraised him and confirmed what he had all along maintained : Anton Grigoriev was not a fieldman. His rapt attention, the unguarded frankness of his expressions as each move was played or contemplated, had an innocence which could never have survived the infighting of Moscow Centre.

Toby's personal appearance was another of those happy chances of the day. Out of respect for the Bernese Sunday, he had donned a dark overcoat and his black fur hat. He was therefore, at this crucial moment of improvisation, looking exactly as he would have wished had he planned everything to the last detail : a man of position takes his Sunday relaxation.

Toby's dark eyes lifted to the Cathedral Close. The get-away cars were in position.

A ripple of laughter went out. With a flourish, one of the bearded players lifted his queen and, pretending it was a most appalling weight, reeled with it a couple of steps and dumped it with a groan. Grigoriev's race darkened into a frown as he considered this unexpected move. On a nod from Toby, Skordeno and de Silsky drew one to either side of him, so close that Skordeno's shoulder was actually nudging the quarry's, but Grigoriev paid no heed. Taking this as their signal, Toby's watchers began sauntering into the crowd, forming a second echelon behind de Silsky and Skordeno. Toby waited no longer. Placing himself directly in front of Grigoriev, he smiled and lifted his hat. Grigoriev returned the smile - uncertainly, as one might to a diplomatic colleague half-remembered - and lifted his hat in return.

'How are you today, Counsellor?' Toby asked in Russian, in a tone of quiet jocularity.

More mystified than ever, Grigoriev said thank you, he was well.

'I hope you enjoyed your little excursion to the country on Friday,' said Toby in the same easy, but very quiet voice, as he slipped his arm through Grigoriev's. 'The old city of Thun is not sufficiently appreciated, I believe, by members of our distinguished diplomatic community here. In my view it is to be recommended both for its antiquity, and its banking facilities. Do you not agree?'