But, he says, “I was doing it for myself if I’d been doing it for the money I would have stopped long ago”. This commitment strikes me as extraordinarily single-minded. He’d be on the 6.30am train from Oxford, and back at the dining room table by 6pm to get 350 words down before the day’s end.

But for the greater part of a decade, until the books took off, Herron – whose quiet, measured speech is inflected by a gentle Newcastle accent – was writing the series at nights, while commuting to London to work as a subeditor on a legal journal based, as it happens, near the Barbican. For someone used to working in solitude, Herron has had an unexpectedly enjoyable time as script consultant – “there’s an awful lot of laughter in a writers’ room,” he says. The Oscar-winning actor Gary Oldman is playing Lamb, Kristin Scott Thomas the steely apparatchik Diana Taverner, and Jack Lowden the disgraced young agent River Cartwright. The books have become critical and commercial successes, and when I pay Herron a socially distant visit, filming is under way on Slow Horses, an adaptation for Apple TV.

Which is also the title of the seventh and latest novel in the series, published next month. These mess-ups and headcases have been banished from the sleek MI5 headquarters in Regent’s Park, London, to the secret service’s institutional oubliette Slough House, a grotty office block near the Barbican. Herron is the author of the hugely entertaining Jackson Lamb series, about a stunningly inept collection of secret agents. He is clearly not lazy, not at all, about the stuff that matters to him. “Obviously there are things I would need to check, and I would go out and do that.” At this point I look at him as if he’s just told me he gathers information by examining the entrails of birds. I don’t like having to learn new things, I’m too lazy,” he says.

He’s still buying CDs (there’s a soaring Gavin Bryars choral work playing on the stereo) and at one point he produces his mobile, which, like the vast Toshiba laptop enthroned on a side table, seems suspiciously elderly.

The other immediate impression given by his home is of – how shall I put it? – antiquity. W ere it not for the packed bookshelves – everything from Len Deighton to the complete Philip Larkin – you could almost imagine the novelist Mick Herron’s flat as a safe house, plain and modest as it is, tucked away in a rather anonymous modern block in Oxford.

Dead lions by mick herron