It divided opinion when it debuted last April but there's a charming, ingenuous quality to the offbeat humour in this new comedy series from Ricky Gervais. He stars as Derek, a vulnerable adult working in an old people's home, who reckons he's the luckiest man in the world, surrounded by all of his 'favouritest' people, including long-suffering best friend Dougie (Karl Pilkington) and the home's manager, Hannah (Kerry Godliman). The laughs are bittersweet and there's a poignant truth beating at the heart of the story - the council is looking to cut its budget, which means Derek's happy home faces the chop.

Last year's pilot episode got a mixed reception, so many were surprised when Ricky Gervais's comedy-drama about a retirement home worker with learning difficulties was commissioned for a full series. Yet here it is, starting a run of six episodes. Gervais writes, directs and stars as sweet-natured, animal-obsessed, autograph-hunting Derek Noakes. Gervais's regular sidekick and stooge Karl Pilkington reprises his role as caretaker Dougie, while Kerry Godliman is the standout performer as workaholic manageress Hannah. This opening episode finds Broadhill care home's future under threat from council cuts. Hannah takes inspectors on a guided tour but her efforts to impress are hampered by tadpoles in the bathroom and a naked guest in one of the beds, leaving the ragtag team struggling to prevent the home's closure. There are tender moments and its heart is in the right place, but the end product is misjudged. Much of the acting is awkward, while the script attempts to wring laughs out of bad wigs, sexual innuendo and gratuitous swearing. You can't help feeling it was Gervais's past reputation and celebrity status which got this made, rather than the quality of the show.

Tasteless? No. Pointless? Possibly. It's hard to believe, after last year's misfiring pilot, that Derek would have got a series commission without Ricky Gervais's involvement.

Shattered nuts and well-used sledgehammers litter the scene of this strange comedy-drama mock-doc: telling trumps showing every time, and the converted cover their ears while the preacher drones on about tolerance over a cloying piano soundtrack.

For the uninitiated, Derek tells the story of the eponymous innocent savant (Gervais), a slightly slow but tender-hearted care worker in a retirement home, misunderstood by the outside world but loved by those who take the trouble to get to know him. In this week's opener, said retirement home is slated for closure by the council. Derek and chums Dougie (Karl Pilkington) and Kev (David Earl) join home manager Hannah (the excellent Kerry Godliman) in taking some direct action.

No one can doubt Derek's good intentions, but it owes its existence to a man desperate to prove he's not who we think he is. And as for the comedy? Well, with apologies to Oscar Wilde, the final scene is so hilariously sappy that you'd need a heart of stone not to laugh.

On Channel 4's opening night in 1982, Ian McKellen starred in Walter, a drama about a man with learning difficulties who tries to make his way in a cruel world filled with suspicion and derision. In Derek (Channel 4, Wed, 10pm), Ricky Gervais stars as a man with learning difficulties who tries to make his way in a cruel world filled with suspicion and derision. And has Karl Pilkington a best friend.

The contrast couldn't be more stark. Whereas the future knight and Lord of the Rings star simply was Walter, Derek is The Office boy with a greasy haircut, bad knitwear and facial tics. The cynical might view Derek as Gervais making a grovelling apology for 'Mong-gate' when he threw a word around on Twitter in late 2011 which attracted the ire of the Daily Mail (obviously), Susan Boyle and MENCAP. Except the writing of Derek was well under way by then ahead of its pilot episode last spring.

Like the overwhelming majority of modern comics, Gervais' heart is solidly in the right place but the brain has a tendency to force a foot deep into his mouth from time to time. Taking risks and making an inevitable mistake or ten is part of the comedian's job description. Here, though, Gervais has gone almost entirely in the opposite direction. Soundtracked by Einaudi, Derek is overstuffed with manipulative schmaltz, and so sickly-sweet that it requires you to undergo an emergency filling just by switching it on.

Shunning the pratfalls of the pilot, Derek is now a conscience-driven series in which besuited health executives visit the care home where the eponymous 49-year-old works, callously poking around to see where cuts should be made or whose jobs can be exterminated. Oddjob man Dougie (Pilkington) is one obvious candidate for the chop, while the delicate situation is not helped by the inexplicable presence of a sleazy waster Kev (David Earl). He brings a certain David Brentness to proceedings, replacing tugging on his tie with slugging on an endless stream of Special Brew while attempting to force himself onto any female (whether old, obese or other) unfortunate enough to cross his awful path.

Gervais' triumphs here are to show that the previously irritating Pilkington is actually half-a-decent actor and to write a beautiful lead role for Kerry Godliman as the stoic care home leader. Where it falls spectacularly down is through some rather lazy button-pushing (especially with the endless photo-montages of aged residents in their youthful pomp) and in Gervais' massively distracting central performance which hinders rather than helps the series. And will he ever give the mockumentary genre a break?

How We Met: Karl Pilkington & Ricky Gervais

Karl Pilkington and Ricky Gervais explain how they first met. Karl says of Ricky: "Now he's my best mate, but in the early years he became obsessed with finding out where I lived."

The pilot last year may have been a bit divisive, but Channel 4 obviously liked Ricky Gervais's Derek enough to order a full series of the bittersweet retirement home comedy - we're just wondering if he can keep that face up for six whole episodes. The Office mastermind's latest project kicks off proper with sweet and simple Derek teaming up with co-workers at the home Dougie (Karl Pilkington, basically playing himself) and Hannah (Kerry Godliman) to save it from closure.

Ricky Gervais in Derek, Channel 4 - review

Despite good performances from Kerry Godliman and Karl Pilkington, the first full series of this care home comedy looks like another slushy mess, says Jack Seale.

I often watch this when I have a half-hour to fill, and it makes me chuckle. Karl Pilkington works better in an audio or animated context than live-action (An Idiot Abroad), although the quality of his anecdotes and left-field thinking has taken a dip this series. You have to wonder how many times they can pick his brains before, well, he runs out of brains worth picking.

This animated rendering of Ricky Gervais, Stephen Merchant and Karl Pilkington's weird podcast ponderings still makes for great late-night viewing. This time around they discuss the Ancient Greeks and what it would be like if Karl was president of society. Heaven help us.

The day before the one-off comedy Derek screened on Channel 4, Ricky Gervais announced on Twitter that he was "slowly writing" a full series.

For a particularly vocal section of the media, that writing probably can't proceed slowly enough. Of all Gervais's creations, Derek Noakes is both the nicest and the most instantly reviled. Never mind that he works in an elderly care home, guilelessly chatting to the residents while clipping their toenails and watching Deal Or No Deal: Derek, we've been told, is a monster.

Or at least his creator is, for daring to invent a character whose regrettable hairstyle and awkward carriage imply some sort of learning difficulty. Or does it? Gervais claims Derek is just someone on the margins of society, falling somewhere between Baldrick and Frank Spencer. But on screen, as he jutted out his chin like Monty Python's Gumby, I felt myself not so much cringe as physically recoil.

The niggling feeling that something is off actually helps sustain the overfamiliar fake documentary format. If a filmmaker did come across a subject as perplexing as Derek, you could well believe they would want to have two cameras on him at all times. And the way Gervais plays to those cameras, flicking his eyes between them, at once rigidly self-conscious and plausibly natural in his behaviour, demonstrates his mastery of a genre he single-handedly propelled into the mainstream. It's hard to imagine anyone else playing this problematic part with as much skill, albeit to such uncomfortable effect.

The real problem with Derek, though, is that it's not that funny. There's a lot of slapstick but - one notably satisfying headbutt aside - it just seems clumsy rather than effervescent. The necessarily drab care home setting also begins to feel oppressive, even during the brisk running time.

But perhaps the most depressing thing about Derek is that the media kerfuffle has completely overshadowed the work of Kerry Godliman, who is heartbreakingly plausible as Hannah, Derek's protective workmate who clearly feels life is passing her by but isn't sure how to achieve any existential traction. I found my viewing experience improved by imagining Hannah was the hero of the piece, with Derek reduced to the role of sidekick. Karl Pilkington and his terrible wig, on the other hand, should probably just stick to travel shows.