I knocked on Dr Thoms's door and it was opened by a burly young man in jeans and a T-shirt – in his late twenties, I would have said – who had a shock of curly brown hair tumbling to his shoulders and an almost painfully neatly trimmed beard, all angles and hard edges.

'Ruth Gilmartin,' I said. 'I've come to meet Dr Thoms.'

'You've found him. Come in.' He had a strong Yorkshire or Lancashire accent – I couldn't tell them apart – 'Coom in' he had said.

We sat down in his study and I refused his offer of tea or coffee. I noticed he had a computer with a screen like a television on his desk. Bobbie had told me that Thoms had written his doctorate on Admiral Canaris and MI5 penetration of the Abwehr in World War Two. He was now writing a 'vast book' for 'vast sums of money' on the history of the British Secret Service from 1909 to the present day. 'I think he's your man,' Bobbie had said, rather pleased with his efficiency.

Thoms asked me how he could help me and so I started to tell him, in the most circumspect and vague terms I could manage, given my limited knowledge of the subject. I said I was going to interview a man who had been fairly high up in the Secret Intelligence Service during the war. I just needed some background information, particularly about what was going on in America in 1940-1, before Pearl Harbor.

Thoms made no effort to conceal his quickening interest.

'Really,' he said. 'So he was high up in the British Security Coordination.'

'Yes,' I said. 'But I get the impression he was something of a freelance – had his own small operation.'

Thoms looked more intrigued. 'There were a few of them – irregulars – but they were all reeled in as the war went on.'

'I have a source who worked for this man.'

'Reliable?'

'Yes. This source worked for him in Belgium and then in America.'

'I see,' Thoms said, impressed, looking at me with some fascination. 'This source of yours could be sitting on a goldmine.'

'What do you mean?'

'He could make a fortune if he told his story.'

He. Interesting, I thought – let's keep him a he. And I had never thought of money, either.

'Do you know about the Prenslo Incident?' I asked.

'Yes. It was a disaster, blew everything wide open.'

'This source was there.'

Thoms said nothing – only nodded several times. His excitement was palpable.

'Have you heard of an organisation called AAS Ltd?' I asked.

'No.'

'Does the name "Mr X" help you identify anyone?'

'No.'

'Transoceanic Press?'

'No.'

'Do you know who "C" was in 1941?'

'Yes, of course,' he said. 'These names are beginning to come out now – now the whole Enigma/Bletchley Park secret is exposed. Old agents are talking – or talking so you can read between the lines. But,' he leant forward, 'this is what is fascinating – and it makes me sweat a little, to be completely honest – as to what SIS was really doing in the United States in the early days – what the BSC was doing in their name – is the greyest of grey areas. Nobody wants to talk about that. Your source is the first one I've ever heard of – from an agent in the field.'

'It's a stroke of luck,' I said carefully.

'Can I meet your source?'

'No, I'm afraid not.'

'Because I have about a million questions, as you can imagine.' There was a strange light in his eye – the light of the scholar-hunter who has smelt fresh spoor, who knows there is an unblazed trail out there.

'What I might do,' I offered, cautiously, 'is write some of it down, in broad outline, see if it made any sense to you.'

'Great. Happy to oblige,' he said, and leant back in his seat as if, for the first time, he were just taking in the fact that I was, for example, a member of the female sex, and not simply a new mine of exclusive information.

'Fancy going to the pub for a drink?' he said.

We crossed the High and went to a small pub in a lane near Oriel and he gave me a potted synopsis of SIS and BSC and the pre-Pearl Harbor operations as far as he understood them and I began to understand something of the context for my mother's particular adventure. Thoms spoke fluently and with some passion about this covert world with its interconnecting lines of duplicity – effectively a whole British security and intelligence apparatus right in the middle of Manhattan, hundreds of agents all striving to persuade America to join the war in Europe despite the express and steadfast objections of the majority of the population of the United States.

'Astonishing, really, when you come to think of it. Unparalleled…' He stopped suddenly. 'Why are you looking at me like that?' he asked, a bit discomfited.

'Do you want an honest answer?'

'Yes, please.'

'I can't decide whether the hair doesn't go with the beard or the beard doesn't go with the hair.'

He laughed: he seemed almost pleased by my bluntness.

'I don't usually have a beard, actually. But I've grown it for a role.'

'A role?'

'In Don Carlos. I'm playing a Spanish nobleman called Rodrigo. It's an opera.'

'Yeah. That Verdi bloke, innit? You can obviously sing, then.'

'It's an amateur company,' he explained. 'We're doing three performances at the Playhouse. Want to come and see it?'

'As long as I can get a baby-sitter,' I said. That usually scared them off. Not Thoms, though, and I began to sense Thoms's interest in me might extend further than any secrets I possessed about the British Security Coordination.

'I take it you're not married,' he said.

'That's right.'

'How old's the kid?'

'Five.'

'Bring him along. You're never too young to start going to the opera.'

'Maybe I will,' I said.

We chatted a bit more and I said I'd call him when I had my summary complete – I was still waiting for more information. I left him in the pub and wandered down the High Street to where I'd parked my car. Some students, wearing gowns and carrying champagne bottles, burst out of University College, singing a song with a nonsensical refrain. They capered off down the street, whooping and laughing. Exams over, I thought, term nearly finished and a hot summer of freedom ahead. Suddenly I felt ridiculously old, remembering my own post-exam euphoria and celebrations – an aeon ago, it seemed – and the thought depressed me for the usual reasons. When I took my final exams and celebrated their conclusion my father had been alive; he died three days before I had my results – and so he never learned that his daughter had got a first. As I made for my car, I found myself thinking about him in that last month of his life, that summer – six years ago, already. He had looked well, my unchanging Dad, he wasn't unwell, he wasn't old, but in those final weeks of his life he had started behaving oddly. One afternoon he dug up a whole row of new potatoes, five yards' worth, tens and tens of pounds. Why did you do that, Sean? I remember my mother asking. I just wanted to see if they were ready, he said. Then he cut down and burned on a bonfire a ten-foot lime sapling he'd planted the year before. Why, Dad? I just couldn't bear the thought of it growing, was his simple, baffling reply. Most strange, though, was a compulsion he developed in what was to be his last week on earth, for switching out electric lights in the house. He would patrol the rooms, upstairs and down, looking for a burning light bulb and extinguish it. I'd leave the library to make a cup of tea and come back to find it in darkness. I caught him waiting to slip into rooms we were about to vacate, poised to make sure the lights went off within seconds of their being no longer required. It began to drive me and my mother mad. I remember shouting at him once: what the hell's going on? And he replied with unusual meekness – it just seems a terrible waste, Ruth, an awful waste of precious electricity.