Disturbed by Melissa's unnoticed death and to stave off his own isolation, he practises archery and unofficially continues the investigation past his retirement, after his colleagues close the case as a suicide.

The occupants of the house are nominally happy: young couple Michael Jenson (Russell Tovey) and the pregnant Vidya Khan (Amber Rose Revah) have just moved in; couple Elaine Markham (Indira Varma) and Peggy Scott (Victoria Hamilton) are designers; journalists Kieron Moss (Steven Mackintosh) and girlfriend Patricia Keenan (Claudie Blakley) are discussing further commitment.

Middle-aged maths teacher Joe Sellers (David Bamber) secretly shares his flat with the far younger Liz Fletcher (Denise Gough), who cooks for him domestically but whose relationship with him is otherwise left vague.

His inability to handle alcohol (shown in flashback to his relationship with Melissa) causes him to attempt sex with Patricia without her consent.

Some years previously Liz had murdered her abusive stepfather and Joe had helped her evade the police by allowing her to live undetected in his flat.

Kieron, ashamed of his past drunkenness and wishing to make things right with Patricia, punishes Adam's own bad behaviour by disowning him.

Vidya believes him but realises what Michael has not: by focusing on his revenge instead of caring for her he has revealed he is too immature to bring up a child and maintain a relationship.

The police arrive and Vidya rises to let them in, but Len bids her stay with what may be his final words: he doesn't want to be alone.

Writing in The Daily Telegraph, Ben Lawrence was impressed by What Remains and said it is aconventional whodunit shot as arthouse cinema.

... it is also a gripping mystery, shot through with an unsettling stillness and clearly under the influence of those much-discussed Scandinavian imports which include The Killing.

[29] Caroline Frost, writing for The Huffington Post said:while Broadchurch and Southcliffe tapped into the collective effect of a crime on a tightknit community, What Remains does the opposite – highlights just how it's possible for people to get lost, and stay very alone, in a big city, where close living conditions no longer mean close neighbours, in fact, often the opposite in our desperate struggle for personal space.

Anybody believing a tale like this is ridiculously far-fetched could do worse than watch the moving and disturbing Dreams of a Life starring Zawe Ashton.

[32] Zoe Williams, writing for Radio Times said "What Remains functions brilliantly as a whodunnit, but has a profundity that keeps it turning over in your head".

[34] Arifa Akbar, writing for The Independent, commented that "not all [British detective dramas] are as creepily compelling as this.

While it based itself, quite simply, around one house and its neighbours, its strength lay in its complex interweaving of lives, along with the growing, rather than diminishing, fear that swarmed the crime scene".

[36] Nathan Bevan, a columnist for WalesOnline said he "rather liked the Columbo-like ways of DI Len Harper and was gripped by the overblown gothic melodrama of the final 15 minutes" and he would be "more than happy to tune into for a second series".

Gabriel Tate called it "the latest compelling serial to add to 2013's remarkable roster ... a pleasure to watch such a well-cast ensemble working through such a tightly crafted narrative, while the undertones of isolation and insecurity will be familiar to most big city dwellers.

[40] The New Statesman said:[the series] is clearly intended to be not so much a whodunnit as a why-oh-whydunnit; its writer, Tony Basgallop (Hotel Babylon, Inside Men), seems to be more interested in the way we live now than in weapons and motives.

You will have gathered that I think his script strains credulity and that he's very lucky indeed his cast includes the likes of Threlfall and Bamber.

[41] Gerard O'Donovan, writing for The Daily Telegraph, saidWhat Remains served up such a thumper it made me very glad I'd persisted through the four, at times glacially slow-moving, episodes that built inexorably up to its inventively baroque denouement.

There's been a rash of highly original crime-drama finales this year (Broadchurch, The Fall, Top of the Lake, Southfield) and this wasn't far short of the best.

[42]The Guardian's Sam Wollaston said "In trying to tie up its ends, others have come loose, so what you're left with is a bit of a frayed, knotty mess.

[44] Keith Watson for Metro said What Remains "was a brave attempt to combine a murder mystery with human drama and, to its credit, there was a genuinely disorienting sense of uncertainty about a closing sequence that packed more action into ten minutes than the previous three hours and 50 minutes.

"[45] Ben Walsh of The Independent gave it four out of five stars and said "David Threlfall excels as the retired detective who can't let this case go, and, up until the bonkers and gothic last 10 minutes, this is beautifully paced TV.