Skellig, Young Vic, London

His breath stinks. He's riddled with arthritis. He gobbles spiders and raw mice and leaves owl-like droppings. Two suspicious humps bulge under the shoulder blades of his coat. His hygiene and dress sense would drive the What Not To Wear pair to early retirement. So not the standard idea of an angel. But then is "angel" an accurate description of this baffling creature? It's a question that is left tantalisingly open both in David Almond's award-winning story, Skellig, and in this engaging but overly sentimental stage recreation, directed by Trevor Nunn as a piece of agile narrative theatre for the Young Vic's celebrated Christmas slot.

Looking like a young Colin Welland playing a short-trousered schoolboy in Dennis Potter's Blue Remembered Hills, Kevin Wathen is just right in the role of Michael, the Geordie lad who discovers Skellig in the garage of the new family home. His dumpy, likeable ordinariness and his air of flustered preoccupation would make anyone identify with the plight of this kid, who is having to cope with the related pressures of moving house and of being slightly sidelined by a prematurely born and desperately ill sister. Resembling the ashen ghost of a dosser, David Threlfall is excellent casting, too, deflecting the boy's amazed questions with croaks of curmudgeonly Northern sarcasm.

The piece is about the power of belief. There's a mystical link between the baby fighting for her life and the terminally disconsolate Skellig, whom Michael gives first the means (the remains of Chinese takeaways and beer) and then the will to live. Though my own children dispute this, I think this fable would be more powerful without Mina, the home-taught, William Blake-quoting girl. The fact Michael allows her to join him in the adventure with Skellig gives the creature, too much external corroboration. It would be a greater test of Michael's faith if he had to credit Skellig alone, besides leaving it mysteriously moot as to whether this being is principally a projection of his personal needs. If Mina comes across as an irksome wise child in the book, in the play Akiya Henry has been encouraged to perform her as a hectoring, hyperactive know-all.

There are some magical episodes. I particularly liked the bit where tawny owls swooped across a red-lit sky, dangling an inch from the audience's noses at the end of long rods. It's hard, too, to resist the moment when the chorus of narrators carries the frail and confused Skellig aloft to his new home. But glutinous pipe and organ music mars the moment where Michael, Mina and their winged friend levitate in a dancing aerial ring of mutual belief. Too often, the show succumbs to the values of a mushy musical. The Young Vic's Christmas shows tend to be spikier and to use fewer resources to be more abundantly inventive.