It used to be customary for sitcoms to change their location when they returned, after the end of the regular series, with a film spin-off or a feature-length Christmas special. The characters would be holidaying abroad, perhaps, or at least loading up the car and going to an unfamiliar part of the UK. Detectorists (BBC Two), one of the very greatest sitcoms, would never do that. It is as rooted in the eastern English countryside as a thick oak. The farthest afield it can comfortably travel is to a neighbouring field. Five years since we last saw them, nothing much is different for middle-aged best pals Andy (writer/director Mackenzie Crook) and Lance (Toby Jones). Their big news is a fresh “permission”, a green light to hunt for buried artefacts on farmland unvisited by detectorists. Their conversation with the landowner, who repeatedly refers to the duo as “metal detectors”, sums up Andy and Lance’s view of anyone who doesn’t share their passion: they tolerate his ignorance for as long as is necessary, but are happy when he has left them alone to survey their new domain. This is a comedy about not going anywhere, for good and for ill. The way Andy and Lance have devoted hours, weeks, years to pacing up and down in mud, waiting for their detectors to beep, is representative of enthusiasts and dreamers everywhere: concentrating on something esoteric helps them to delay confronting tricky real-world issues such as relationships, careers and emotions. But the show is also about recognising that everything you need in life might be right in front of you, from the loving partner who tolerates your silly obsessions, to the green hills at the edge of your humdrum town that are as beautiful as any far-flung vista. A quiet afternoon in the clean air and a pint of local ale in the pub with a friend afterwards is reward enough. It is a pure and humble pleasure, then, just to go back to that sleepy paradise, somewhere between Essex and Suffolk, to reacquaint ourselves with the full-hearted eccentrics of the Danebury Metal Detecting Club. DMDC president Terry (Gerard Horan) has painstakingly authored a talk on “the pros and cons of calibrated balance versus pulse induction when coping with ground mineralisation”, which he will happily deliver for the club’s six members. One of the other epiphanies Detectorists diffidently nudges us towards is that humanity is made of these small commitments, just as every arrowhead and broken belt buckle the detectorists dig up adds to the grand patchwork of history. Before the talk, Lance has offered to “knock out a jalfrezi” for himself and Andy, and we catch him preparing the meal with the radio blasting, delivering the chorus of It’s Raining Men in the manner of someone who predicted that exactly these weather conditions would occur. All is well. But it is also traditional for belated comeback specials not to be as good as the original series, because the balance of the storytelling is off – and, although a bad episode of Detectorists is still the best thing you will watch all Christmas, the delicate equilibrium it maintained through three seasons of elliptical half-hour episodes isn’t quite kept up. Andy and Lance discover evidence of a major historical battleground on that farmland, prompting excitement about the one huge find they’ve always been waiting for; briefly, it looks alarmingly as though it might turn into something akin to the climax of Only Fools and Horses. The show already delivered the perfect poetic payoff at the end of season three, when gold coins secreted in a magpie’s nest rained down. This is a comparatively vulgar raking of the same ground, and the rift the big news causes between the pair is, by Detectorists standards, as unbalancing and alien as it would be if Crook packed the whole cast off to Torremolinos. Storylines for supporting characters that would have flourished in a full fourth season struggle to get a foothold in a 75-minute special. Detectorists is never shy of showing how Andy tries the patience of his more assertive wife, Becky (Rachael Stirling), but some scenes early on, where a setback in their lives causes her to demand once again that he buck up his ideas, write cheques that the denouement doesn’t cash. The show’s ability to generate piercing sadness is absent: it seems there are no more smothered secrets to reveal. And yet the performances are still deft and generous. The cinematography – this time capturing a hare, a hedgehog and a white fallow deer wandering by – is still like the work of master English landscape painters brought into three dimensions. The jokes are still sharp, dry and wise. At sunset, Detectorists still shines a precious warm light.