What would we do without Billy Connolly?

That's a rhetorical question, yet given the fact that the much-loved comedian and actor, now 71, has prostate cancer behind him and Parkinson's to contend with, it's also a question that adds an unavoidable resonance to his latest film.

What We Did On Our Holiday may be a comedy, centred on a trio of delightful performances by children, but Connolly's presence as a paterfamilias facing death through cancer adds depth and resonance to proceedings. He also happens to provide a performance of wit, warmth and wisdom, which anchors an otherwise so-so outing.

Doug and Abi McLeod (David Tennant and Rosamund Pike) are separated, and expecting to divorce. There seems to be little left in their relationship other than vitriol - and their three kids are caught in the middle of their endless bickering.

The pair agree to a truce so that they can drive from London to Scotland for the birthday celebrations of Doug's father, Gordie (Connolly). Torn every which way by their divorce lawyers, the couple are taking damage limitation to heart; while asking their kids to keep their troubles a secret from Doug's family, they also conceal Gordie's ill health from the kids. The weekend, then, will be an exercise in secrets and lies. The joke that drives the film is that the kids have no intention of playing ball.

Five-year-old Jess (Harriet Turnbull) is a scream, who keeps a brick as a pet and will happily steer the car behind her Mona Lisa sunglasses when dad falls asleep at the wheel; six-year-old Mickey (Bobby Smalldridge) is eccentric, self-contained and obsessed with Vikings; the oldest, sensitive nine-year-old Lottie (Emilia Jones), is the most cognizant of the family crisis, her decision to make a list in her diary "of the lies we're going to tell" a telling riposte to her parents.

When they reach the Highlands, there is even more ridiculous adult behaviour, in the form of Doug's obscenely wealthy, materialistic and anal brother Gavin (Ben Miller) and his unhappy, kleptomaniac wife Margaret (Amelia Bullmore).

The sanest of the grown-ups by far, and the best company for the kids, is the one who ought to be the most miserable. Connolly plays Gordie as that dream adult for a child, one with an empathetic rapport with each of the children individually and a laissez-faire approach to the group as he smuggles them out for a day trip that easily wins them over. It's rare that Connolly doesn't invest some of the irreverence of his stand-up into his acting; here, Gordie's ageing rebellion is an obvious act of defiance against the inevitable, which makes the film's final phase incredibly moving.

Tennant is fun as a father who never quite manages to find enough substance, Miller good as his uptight sibling, and the kids a hoot. But Connolly steals the show.

The writing and directing team of Andy Hamilton and Guy Jenkin are best known for their TV sitcom Outnumbered, on which they pitted adult actors working with a script, with child actors encouraged to improvise. They bring the same approach to their first film, where it works a treat, with some of the youngsters' priceless lines seeming to flummox the adults for real.

Elsewhere, their sitcom roots are too evident, with the character dynamics and humour coming across as forced and twee. It's very sweet, but when Connolly is off-screen, the hectoring and determined kookiness can wear a bit thin.