This isn’t a book to read before lights out. It’s about a mentally ill man whose mother exiles him from rural Ireland after years of rumours and reprisals related to his habit of startling passers-by with his bared erection. She has tried strapping him to a chair and bolting the door, but all that did was give him a fetish for not emptying his bladder. Now Martin John is flat-sitting in south London, working as a nightwatchman and hoarding old Eurovision tapes and lists of words beginning with P. But menacing this toehold on equilibrium is the arrival of an ill-disposed male lodger, who swiftly becomes the object of his paranoia.

Martin John’s fevered brain (it’s always ‘Martin John’) gives the novel its jagged rhythm. Often we can’t make sense of what we’re reading — why does he keep mentioning Beirut and asking people to ‘check his card’? — but these and other refrains crystallise with revelations of past crimes. We’re shown how he masturbated before calling 999 when he discovered a previous lodger (a woman) comatose in bed after a suicide attempt; the look from the paramedic convinced him only to rent to men in future. The ominous narration, both protective and accusatory, is privy to these thoughts but not wholly aligned with them: it’s like the voice Martin John might hear in his head.

Somehow the protagonist’s extreme aversion to using the toilet adds to the agitation. The pressure builds as we piece together how he got to where he is now — a process aided by a shift in perspective to the victim of his worst offence. His fixation with his new lodger adds a thrillerish voltage, as he embarks on a guerrilla eviction campaign involving crank calls, sabotaging the plumbing and branding his items with a £26 custom-made ink stamp: ‘If you remove or move this object or currently have this object in your hand you have violated code 1066.’

The fizzy verbal surface serves to sugar-coat a grown-up tale of how blighted lives carry on. While there’s late comic relief from a massive delay Martin John causes at Euston, his favoured haunt, it doesn’t herald a happy ending. This is a book about social breakdown as well as mental breakdown, with a low-key portrait — almost in passing — of a no-questions-asked migrant labour market in which Martin John can be tolerated but not helped. Ultimately it’s about that £26 stamp. Imagine selling that — you’d know something was up, but what could you do? We recoil from the response of Martin John’s mother — but what can she do? The novel won’t pretend it’s easy.