Despite the cosy room, she understood that this was both a briefing and a test: if she failed now, she would go no further.

‘I understand.’

‘GCHQ is the largest and most professional secret organization in the world, and consists of this: layer upon layer of subterfuge. All right?’

‘Yes, sir.’

‘Our first layer of defence is that we don’t exist. Our second layer is compartmentalisation. Many of the people here on site – it’s eighty per cent women, by the way – know little of what happens outside their immediate function. For example, the strategic meaning of decrypted intercepts passing through their hands.’

Gavriela nodded. This was why she was here: to wage a secret war. Also, she had underestimated the percentage of female staff, which was interesting.

‘Our third layer,’ Frank went on, ‘is the illusion of ephemerality, something knocked together in the usual haphazard English way in response to the Nazis.’

He smiled, drank tea, then looked at Gavriela.

‘The truth is,’ he said, ‘GCHQ is in its fourth decade of operation. You could say its raison d’être was confirmed in its fifth year, when the first captured German naval codebooks were handed over by Frenchmen to the Royal Navy, enabling thorough decryption of the most vital signals. The rendezvous took place in 1914, and the receiving British officer’ – he pointed to the photograph – ‘was a gentleman called Winston Churchill.’

Her spine tingled.

‘If enemy agents penetrate that far into our secret layers’ – Frank’s voice hardened, all languor gone – ‘then the next subterfuge is that we work solely on something called the Enigma code, which you will be briefed on later. There are four other main areas of ciphers and intercepts that we work on. At least two of them outweigh Enigma by a considerable margin, in their importance to the war effort.’

Frank rubbed his moustache.

‘One of those areas is subdivided into three sub-projects, one of which you’ll be working on. What they have in common, the sub-projects, is their concern with German teletype codes. Recent intercepts include,’ he added, ‘signals to and from the Führer himself. So far we’ve not been able to break anything at the time of sending. Nevertheless, three-week-old news is rather better than none at all.’

Gavriela swallowed. Already she was party to privileged information she could never hint at.

‘You don’t need to know what the other areas are,’ Frank finished. ‘At least for now.’

‘Do you issue cyanide capsules?’ she asked, not knowing if she meant it.

‘We’d rather you don’t get exposed to the possibility of capture,’ said Frank. ‘Parachute drops into occupied territory are for the SIS boys – and they are based here at BP, alongside ourselves – plus SOE. But not us.’

That was just as well, because Gavriela could be more effective with pencil and paper than with physical action.

‘But that brings me,’ Frank continued, ‘to the last layer of subterfuge.’

‘What’s that?’

‘Given our somewhat academic background, anyone who learns of GCHQ’s existence is likely to think of us as boffins and nothing more. Naive and dithering, the stereotype of the English academic. It provides useful cover. But the truth is, everyone needs to learn a little tradecraft.’

‘What does that mean?’ asked Gavriela.

‘We’re going to train you to notice everything. Any hint of enemy surveillance beyond these grounds, we expect you to pick up on instantly. The slightest thing out of kilter when you’re travelling, you’ll learn to escape and evade and go to ground.’

Gavriela nodded, her throat tight.

‘You won’t be a commando, but you’ll have go through the course that includes learning how to kill’ – Frank held up a fountain-pen – ‘with this into someone’s throat, for example. If that’s what’s required to get clear. Frankly, we don’t train long enough for it to become reflex, and we’re certainly not silent-killing types. But you never know.’

‘I … all right.’

‘The main thing is spotting if someone is following you. If you suspect someone is a fifth columnist or simply too nosey about your work, no matter where you are, you’ll have a telephone number to call. The result is the party in question will disappear.’

A privilege with consequences.

‘We’re serious here,’ Frank added. ‘The question is, are you on board?’

‘I am.’

Frank held out his hand.

This is commitment.

A solemn oath, their handshake.

‘Civilization,’ Frank said, ‘is fragile, requiring protection.’

‘I’ll give my life if I have to,’ said Gavriela.

After eight weeks of training and helping out in Hut 6, she began the new year by moving to the virtual Hut 27 – of no fixed physical location, but temporarily in Section F – where the new team introduced themselves.

Clive was a mathematician, a musician, and something of a wet drip according to Harry and Fred. A perpetual underlying odour of chilblain ointment emanated from his desk, and he always found something to moan about while they toiled in a safe hut warmed by endless cups of tea, while around the world thousands were dying by the day.

‘We’re an offshoot of Bill Tutte’s team,’ he told Gavriela. ‘Young Bill worked out how the Tunny machines operate purely on the basis of intercepts. It’s at least as impressive as anything AMT has done’ – he meant Turing – ‘and the Tunny signals are orders of magnitude more important than Enigma, whatever they might have told you in Hut 6.’

Perhaps Clive knew what he was talking about. He had worked first on breaking the Enigma codes: hence, perhaps, his habit of humming out intercepts in Morse while he worked on them, even though the teletype codes were a 5-bit encoding and transmitted as such.

Then there was Harry, with his narrow Errol Flynn moustache and over-use of Brylcreem – he kept a tube in his desk drawer – always the self-styled charmer of the group. Having spent time in the Foreign Office before returning to teach modern languages at Durham, he considered himself wise in matters of strategy in a way that annoyed the others. But he was bright, and had taught Clive to correct his mangled pronunciation of German words in quite a creative way: he encouraged Clive, as a pitch-perfect musician, to sing the syllables.

Fred and Olivia were similar opposites: a large Cantabrigian and a thin Oxonian, mathematicians who viewed languages as logic systems – they sometimes argued in dry-sounding classical Greek – and both incessant pipe-smokers. (Olivia would sometimes stare at Gavriela in ways that made her uncomfortable for several weeks; then she accepted it and nothing happened.)

Silvester was the elder statesman of the group: a professional intelligence officer who had carried out similar work in the Great War (and was responsible for recruiting Clive and Harry), he could occasionally be persuaded to reminisce about the cracking of German naval codes and the difficulties then of persuading the military to act on signals intelligence.

‘Can’t blame them, really,’ he said. ‘Partly because they had no idea what Room 40 was, and partly because we had the same problem as SIS with human intelligence: had we revealed our sources to show our credibility, we would have compromised those sources. Revealing an agent’s name or which cipher one has cracked are equally damaging.’

Room 40 in the Admiralty had been one of the roots of the fledgling GCHQ, with at least as much codebreaking success then – a quarter of a century ago – as now.

The remaining four codebreakers were Brian – handsome, one-armed, who had been staying in the same Oxford digs as Gavriela when they met – plus Harriet, June, and Sophie, all with cut-glass accents, impeccable manners and ferocious focus once they had a problem in their grip. The three women belonged to the social class Rosie had described on Gavriela’s first day; and they were nice enough.