Visiting home, even long enough to fetch and park the car, was dangerous enough, though he doubted if there could be a police call out for him and Betty yet. Staying there (quite apart from their determination to be out of town as soon as possible) would be out of the question; Beehive, he knew, took very prompt action against defectors and by this afternoon at the latest the local police would be alerted.

'Which way now?' Betty asked. 'We can't cross the Canal. It's either Lea Bridge Road or the railway.'

The Canal ran parallel to the Lea river, two or three hundred metres ahead. Crossing it by the road bridge would be almost direct but Philip thought it was better to avoid it. They swung north across the playing fields towards the railway bridge a kilometre away.

After a minute or so, Tonia stopped, her head on one side. 'Did you hear that?'

'Hear what?'

'And feel it. A sort of rumble.' 'Train coming, perhaps?' 'No-I…'

Even as she spoke, the great shock-wave threw them off their feet. If they all screamed, the sound was lost immediately in a vast thunder of noise. A fissure suddenly appeared in the ground beside them, widening and shooting out lengthwise as they scrambled away from it. 'Keep together, for God's sake!' Philip yelled. They were still on all fours, unable to stand on the jerking, groaning earth. How long it lasted, they never knew; ten seconds, half a minute… They were too overwhelmed with impressions -the railway bridge ahead of them twisting and tilting like soft wax, the crash of falling buildings on each skyline, the terrible metallic clatter of endless multiple pilc-ups on Lea Bridge Road… the deadly grey mist seeping up through the nearby fissure and Tonia crying out 'Respirators -quick!'…

They fumbled with the black rubber muzzles, holding their breath while they pulled them over their mouths. Now we're marked game, Philip thought dizzily – the law of the jungle.

They gazed at each other, stunned by what had happened. It was some seconds before they realized that the world around them was, briefly, almost silent.

And after the silence the screams began. Screams mounting and multiplying from the carnage on the roads; screams, more distant, from the shattered bricks and concrete ahead and behind, where the still-living were trapped and maimed, and the lucky (lucky?) uninjured struggling to free them. In three days they'll be mad… Philip thrust the thought away in horror.

'The river-look!'

The two women turned at his mask-muffled voice and looked. Behind them, surging and tumbling, the River Lea had burst its banks and was rising inexorably towards them.

They took to their heels, watching for fissures as they ran. Philip led them obliquely northwards, in a race to reach the twisted railway bridge before the flood did. They made it with water lapping about their ankles and scrambled to immediate safety among the ruined girders. Below them, by some freak, the Canal bed had drained dry. A giant, Dust-spewing fissure diagonally across it was a possible reason but already the floodwaters were cascading over the wall as though trying to refill it. They whole thing was a Titan's beach-game, transforming in minutes a landscape that had been familiar to Philip since he was a boy.

He tore his eyes away from it and examined their position as coolly as he could.

The bridge was an X-shaped one on two levels, the Clapton-Walthamstow line crossing above the Tottenham-Stratford one and at right angles to it, each at forty-five degrees to the Canal. Both were now a single mass of wreckage, but the south-easterly line, towards Lea Bridge station and Stratford, looked walkable, and protected by the Canal bed from the floods; the water still seemed to be disappearing into the great fissure. It lay in the direction they wanted to go, so they climbed over the wreckage and followed it.

On the way, they kept their eyes open for weapons. They found a solid iron bar, the rusting front fork from a bicycle and a heavy shovel; with these in their hands, they felt safer. Philip worried about the bridge at Lea Bridge station, where the line passed under the crowded holocaust of a road; would they still be able to get through? The station itself was a relic, long closed down, but…

The bridge, when they reached it, had collapsed, but there was still a jagged tunnel of daylight to one side. They approached it carefully, hiding in the empty station, for there were people shouting and running above and still the wails of pain and helpless terror from the injured and the trapped. Once through the gap they ran – not merely to escape the crowds but because to their immediate left the gasworks were burning fiercely and there might be more explosions at any moment.

Their house was in Church Lane, a few hundred metres away across Leyton Marshes recreation ground and through Marsh Lane. It was not until they were in Marsh Lane itself that they came face to face with trouble.

Four youths saw them and one yelled out: 'Bloody Beehivers! Get'em! Get their masks!'

The four rushed them together – the leader holding a knife as though he knew how to use it. Philip did not wait but made straight for the leader, swinging his iron bar, to put the women behind him. The leader slashed, jabbed, and dodged and once nearly reached him; then a full swing from Philip's bar caught his arm, probably breaking it. He fell to his knees, squealing, and Philip swung the bar again, down on to the youth's skull, feeling the bone shatter.

Philip spun round; Betty had already half-blinded one attacker with a jab from her bicycle-fork and Tonia was wielding her shovel like a battle-axe, holding the other two at bay. Philip disabled one with a blow to the knee and the survivor backed away, wild-eyed.

Philip shouted, 'That's enough – run!' The survivor made no attempt to follow.

Five minutes later they were inside their house, changing hurriedly into civilian clothes – including what Betty could find to fit Tonia. The back wall of the house had fallen away, bringing half the first floor with it, and there was not a window left unbroken. Fortunately the car was still in the lock-up shed round the back and undamaged. They loaded clothes, bedding, tinned food, tools into the car with little time for careful selection and within half an hour they were away, picking a hazardous route along ruined back-streets, eyes open for fissures, back-tracking often, the smell of burning houses in their nostrils, and all the time the crying and wailing, now mostly pitiful rather than terrified. They saw many people but were mainly ignored now that they were out of uniform. They had wrapped scarves round their heads to hide the official respirators – which did not look strange, because everyone who had been able to get hold of vinegar, and many who had not, had muffled their faces in one way or another.

Nobody mentioned the fight in Marsh Lane until they had been travelling a couple of hours and were well out into the Forest near Theydon Bois. Then Philip said dully, through the muffle of his respirator: 'I killed one of them, you know.'

'If you hadn't,' Betty said, 'they'd probably have got our respirators, and then we'd all seven have gone mad. Because it was too late for them – they'd already breathed it. Don't brood on it, darling. I put an eye out, too – and killing him would have been kinder, in the long run.'

'And we may have to again,' Tonia put in. 'I don't like it, either – I've never killed anyone, though I damn nearly did back there. But we didn't set this up – and if we're going to survive, there may be other times when it's them or us. Especially when God knows how many people start going crazy. Because if one in ten was ready with a vinegar mask by this morning, I'm a Dutchman.'

Philip nodded and then asked: 'Well, girls – we got out, somehow, and just in time. What do we do now?'