I suddenly heard voices in the hall. It sounded like Telauga. I wondered if Malevin knew about the forgery. He must – and yet the sophistication of the material seemed beyond him. Could Malevin be a victim of the hoax? It seemed unlikely. There was something not right about the whole enterprise. I knew more about Dagestan than Malevin probably suspected. I knew it was a benighted republic full of Islamists and sheep rustlers. The idea that Malevin had made his personal fortune dealing rare books rather stretched credulity. For the first time, it dawned on me that I had uncovered something that might put me at personal risk.

Working as quickly as I could, I returned the pages to the folders, the folders to the storage boxes and then placed the boxes on the trolley at the far side of the room. As I did so, the sounds on the stairs grew louder. This time they were two male voices: Malevin’s tenor and a gruffer deep bass. The two men paused outside the door of the drawing room, talking animatedly in Russian.

I sat back down at the table and composed myself. I put the torch back in my pocket, smoothed my hair, and then took out a lens cloth and began polishing my magnifying glass. When I looked up, I noticed with horror that a loose page from the diary had fallen out and was lying among the genuine letters.

The door swung open. ‘Did you satisfy yourself?’ Malevin asked. Beside him was standing a bull-necked, squat, muscular man in a boxy double-breasted suit and extraordinarily long, pointed Italian shoes. The man walked with a muscle-bound waddle and was holding a gold mobile phone.

‘It was a privilege to examine this material,’ I said in a tone that I hoped sounded sufficiently awestruck. Malevin beamed, revealing a set of sharp white teeth and a slightly vulpine overbite.

‘I’m pleased you say so,’ Malevin said. ‘This is my associate, Mr Bykov,’ he added. ‘Mr Bykov, this is Dr Slopen, a famous scholar and literaturoved.’

Mr Bykov acknowledged me with a nod. In Russian, he told Malevin that I was puny enough to be a genuine intelligent – the Russian word that covers a range of meanings from intellectual to member of the chattering classes. ‘Mr Bykov is saying you have the physical characteristics of a real intellectual,’ said Malevin, boldly spinning the insult into a compliment as Bykov stood beside him, rolling his neck and shoulders with the loose menace of a wrestler.

‘Ya ponyal,’ I said. ‘Nemnozhko govoryu po-russki.’

Bykov’s snap to attention made me glad I hadn’t waited any longer to reveal that I understood the language. I didn’t want them to say anything compromising in my hearing. It was a lazy asssumption of Bykov’s to think that I couldn’t understand him.

Malevin felt obliged to massage Bykov’s words further. ‘What he is saying is in fact correct because anyone can have muscles, but to train the mind is a much longer process. Old Soviet Union was understanding this. My family too are intelligent. My father was famous academician.’

‘I expect that’s where you got your love of books,’ I said. I had manoeuvred my body to screen the rogue page from the two men. There was no hope of returning it to its proper place with the rest of the diary, but perhaps I would get a chance to slip it in a folder with some of the other letters.

Malevin flashed another foxy grin. ‘In some sense, you are right. And are you perhaps Jewish?’

‘No,’ I said. ‘Are you?’

Malevin laughed, as though instead of merely batting back the question I had made a hilariously implausible assertion of my own. He translated the question for Bykov, who found it funny too.

‘We are laughing,’ said Malevin, ‘because both Mr Bykov and myself are very typical Dagestani men. Very typical physical type, especially for our ethnic group. So for you to say this is a funny suggestion.’

I looked from one man to the other; from Malevin with his slight build and effete hair to Bykov beside him with his dorsal muscles built up into huge meaty flanges. ‘You’re both typical Dagestani men?’

‘Yes!’ said Malevin. ‘Typical Avar, but one ectomorph, one mesomorph – not same body type. You understand? And because, as you said before, we are a mountain people, Avari are a very pure race. Soviet racial science defined the Avars as purest example of Caucasian type. Very long-living people, because genetic endowment very strong on the whole, but also having predisposition for certain disease because base of DNA not wide like in your country with many invasions and immigration.’ Malevin flicked his hair out of his face with his hand. ‘Now, if you have finished your work, we shall put papers away and have a drink?’

‘I’d love to,’ I said, ‘but I still haven’t had a chance to check that letter we were talking about.’

‘Of course. One moment.’

Malevin crossed the room and riffled through the boxes on the trolley. As he did so, Bykov’s phone cheeped, and in the second it took him to turn his back and answer it, I slipped the loose page in between two sheets of acid-free tissue paper in the folder of letters nearest to me. Having done that, I felt the immediate danger had passed. I could simply profess ignorance if it turned up, and claim I’d been less than thorough in my examination of the letters.

‘Here it is.’ Malevin opened the folder and laid the letter in front of me.

It was, as I had suspected, one of the bogus documents; correctly laid out in terms of the period’s conventions, which were largely governed by the necessity for squeezing as much as possible on a single page to save on postage – paid, in the eighteenth century, by the recipient – but wholly unpersuasive in terms of the paper it used and the way it had been aged. And yet, even knowing it was a fraud, I was moved again by the pathetic terms of the plea. Estranged from Hester Thrale since her decision to marry an Italian music teacher, Johnson was trying to rekindle the memory of their friendship, begging for her help as he sank towards insanity and death. Writing these words, he may have had only days to live.

I had to check myself. It was a mirage. The figure faded again – and yet, the powerful sense remained that the words were sincere, that behind the ink was a suffering heart. It was an extraordinary deception, simultaneously elaborate and shoddy; as mystifying in its shortcomings as in its strange potency, and clearly beyond the linguistic talents of Malevin and Bykov to pull off.

‘Please to stay and have drink with myself and Mr Bykov,’ Malevin said, as he returned the letter to its home.

‘I would love to,’ I said. ‘But I need to get home early.’

‘On another occasion then.’

Malevin saw me out himself. As we walked downstairs, he quizzed me about my Russian – I learned it at school, choosing it because it was wholly new to my peers and the only subject I wouldn’t have to play catch-up in – and I asked him what had sparked his interest in English literature.

‘From childhood I loved these writers,’ Malevin said expansively. ‘Byron, Shelley, Shakespeare, of course – you know Pasternak translated? Johnson I am admiring more and more. His dictionary is very impressive. He is like English Lomonosov. You know? Lomonosov our Russian Johnson, but perhaps more talent because also scientist.’

‘Our Russian?’ I said, emboldened by my proximity to the front door.

Malevin caught the nuance instantly. ‘This is an old Soviet habit that I have not eradicated. As you know, Dagestan was part of Russian Empire even in Czar days. Russia export to us Lomonosov, Pushkin and Dostoevsky. In return, they take all Dagestan oil and mineral wealth.’ Malevin opened the door onto St James’s Square. ‘So, if I say “our Russian Lomonosov”, it’s because my people have paid a lot of money for him.’

There was something unmistakably hostile in the way he said the words, as though he had decided to forgo the polite theatre that had characterised the visit and to leave me in no doubt that I was dealing with a man of great will and acuity. The point was underlined when I looked back across the square from beside the steps of the London Library and saw Malevin waving to me from the portico. In the context of our last exchange, the location and the surreally vast house, the gesture seemed not only an assertion of wealth and status but also, even if only subliminally, a threat.