When it comes to one-off family dramas for Christmas, two things are pretty much guaranteed. They’ll begin with credits announcing a starry cast, and they’ll end with a redeemed character gazing at some falling snow as the music swells.

The only tricky bit, then, is what should happen in-between. Should the redemption take place against a backdrop of vaguely gritty realism? Should plausibility be a consideration, or can the writers just rely on the magic of Christmas to get them out of any plot-related trouble? If Santa’s involved – as he so often is – should the show believe in Father Christmas?

In the case of The Heist Before Christmas – set in Northern Ireland – the respective answers are ‘up to a point’, ‘I’ll get back to you on that one’ and ‘er…’. Not surprisingly, the result is a programme that feels as if it’s assembled all the necessary elements but never decided how to put them together.

Should plausibility be a concern – or can the writers rely on Christmas to get them out of plot-holes?

In one mild but rather interesting departure, the person in need of redemption here is not a grumpy old grown-up. Instead, 12-year-old Mikey (brilliantly played by non-starry newcomer Bamber Todd) immediately establishes his meanie credentials by burning down the school Christmas tree, spraying the words ‘Santa is dead’ on a wall as some Brownies walk past and unplugging the power from his town’s al fresco Christmas party. As his rampage continues – and the cries of ‘You little…’ from his victims intensify – we also see James Nesbitt in a Santa costume robbing a local bank. Finding himself cut off from his getaway car, the robber runs into the woods, seen by no one except Mikey.

At which point, we take a break for a spot of that gritty realism. Back home, Mikey has to babysit his seven-year-old brother Sean while his single mother Patricia (Laura Donnelly) works a night shift at a pound shop with a duly Scrooge-like boss.

So how will she ever be able to afford the Christmas bike that Sean is sure Santa will bring? Aware of the problem, Mikey suddenly reveals a softer side, if not a wholly practical one. He will, he decides, head to the woods and offer to show the robber an escape route in return for half of the £80,000 stolen from the bank. Except that when he gets there, the Santa he finds is one with a Norwegian accent (Timothy Spall) who explains that he’s fallen from his sleigh and that Rudolph will be back for him any time now. As Mikey ponders this, the robber turns up and proves oddly unwilling to share the loot, preferring to kidnap Mikey at gunpoint.

From there, the road to the final snow-gazing scene is a frankly tortuous one, with the characters displaying a weird ability to change their personalities to fit whatever the plot demands. Nesbitt, for instance, is sometimes kindly, sometimes socially aware and sometimes a cackling pantomime villain.

And there’s a further confusion too – as the script seeks unavailingly to reconcile the true, non-materialist meaning of Christmas with the need to make sure everybody gets lots of lovely things. (The same, mind you, is true of A Christmas Carol: discuss.)

Also on Christmas Eve, we get the latest installation of A Ghost Story for Christmas written by Mark Gatiss. Unusually, this is adapted not from M.R. James, but from an Arthur Conan Doyle tale, ‘Lot No. 249’, that marks the first appearance in fiction of a violent Egyptian mummy.

Unlike The Heist Before Christmas, this is a drama that knows exactly what it wants to do: to pay respectful and affectionate tribute not just to the original tale, but to the ghost-story adaptations that were such a feature of 1970s Christmases – and so of Gatiss’s own childhood.

Which is to say that it plays it absolutely straight, with not a trace of updating in the quest for contemporary relevance (or whatever). After a ferocious knocking is heard, the first three lines of dialogue are ‘Let me in, for pity’s sake’, ‘Great Scott, what’s the matter?’ and ‘Brandy, get me brandy!’. (Soon afterwards, there’s even a ‘You’re shaking like an aspen leaf’.)

We then flash back seven weeks, to when the chaps at an Oxford college are starting to have their doubts about their colleague Bellingham and his habit of ‘experimenting with arcane bits and bobs’. Or as one of them tells him: ‘Your filthy Egyptian tricks won’t answer in England.’

Gatiss does make a couple of tweaks to the Doyle, introducing a character who might or might not be Sherlock Holmes (but probably is) and making the ending more sinister. Both of these, though, seem like neat additions, and perhaps even improvements. Either way, this is a highly civilised way to spend half an hour – and possibly best served with mince pies and a glass or two of sherry.