It is a chastening thought that Boris Johnson’s responsibilities now include MI6. Alan Judd’s latest novel is particularly interesting about the relationship between our intelligence services on the one hand and our politicians (and their special advisers) on the other.

Deep Blue is the fourth of his spy novels to have Charles Thoroughgood as its central character. (Charles also appears in Judd’s very first novel, A Breed of Heroes, but as a young army officer in Northern Ireland rather than as a spy.) He is now running MI6, a thankless job, particularly as the service is fighting for funds and (worst of all) cast out of central London to an office in Croydon. He’s growing old, too, an analogue spy in a digital world. Paradoxically, this is one of his professional strengths.

When the director general of MI5 picks up a menacing snippet of internet chatter about ‘Deep Blue’, the phrase sets off an echo an Charles’s memory: not to IBM’s chess-playing computer but to an inconclusive MI6 case from 30 years before. It involved a Russian mole who had heard rumours of subversion within the UK involving ‘Deep Blue’, whatever that meant. The only record of it is a paper file, to which Charles has access.

The narrative divides in two: one strand delves into the past — into Charles’s relationship with the mole and the Russian defector who makes the connection between them possible, and with his investigation of British activists; the second strand picks up the investigation in the present, where Charles is hampered by the illness of MI5’s director general, and also by an intimate connection between the leading British activist and the Home Secretary’s special adviser, a woman who exerts an unhealthy control over the now leaderless MI5. To add to the confusion, the activist was also involved in the earlier case, and his sister was once Charles’s girlfriend.

Charles, past and present, is a marvellous character, with an unflappable competence that pulls him through the most unlikely situations. We see him puffing up the service stairs of the Georges Cinq in Paris with a dead Russian defector in a fireman’s lift; a few chapters later he is dealing with a visiting Greek spook who develops an intense attachment to an elderly stripper in Hartlepool. He’s equally adroit at navigating the murky currents of contemporary Whitehall, where the hazards include departmental rivalries, ambitious politicians, maverick spads and a lack of resources.

Then there’s Deep Blue itself, the codename for something that may well be an all-too-possible terrorist threat if a recent news story is to be believed. Unfortunately, we shan’t be able to rely on Charles Thoroughgood driving off in his elderly and extremely conspicuous Bristol to deal with it.