There’s a strong sense of trying to find a new His Dark Materials, or Coram Boy, in Mark Ravenhill’s dutiful adaptation of Terry Pratchett’s fleet but facile novel of democracy under pressure in the parallel world of a South Pacific island.

The ingredients in Melly Still’s fitfully spectacular production are promising enough, with the exploratory British boat washed up on a devastated island of dead ancestors and flying dolphins: a flurry of leaves rise to the top of the Olivier and are instantly transformed into film imagery.

Then the boat in miniature bobbles on plastic sheeting before depositing the imperial party of The Tempest-like survivors in a heaving terrain of grass-skirted natives led by Gary Carr’s lithe, near naked and dread-locked Mau.

Ermintrude from Wiltshire, whose father is 139th in line to the throne of England, morphs into Daphne and is sucked into a Polynesian culture and blossoming friendship with Mau while her disapproving parrot, Milton – Jason Thorpe’s still and startled performance is a downbeat delight – mutters rude words on the sidelines.

Emily Taaffe’s Daphne has to act as a midwife and suck milk from the udders of a huge warthog to feed the chief’s baby: this little Twinkle is represented by a picaninny puppet who sprouts a little winkie after the interval in line with the phallic inconography of the islanders.

The story-line is stuttering by the time there’s a breakaway insurgence led by Paul Chahidi’s vengeful butler, resulting in a fight with Mau in a small boat threatened by waves and sharks: a tumble overboard is followed with a downward plunge on the big transparent screens that surround the design by Still and Mark Friend.

The trouser men arrive in the shape of the new king’s party – a Russian plague has wiped out the other 138 pretenders – and the love story is poignantly subsumed in a sense of nationhood.

There are brisk contributions from Michael Mears as a spindle-shanked baddie, Gaye Brown as an imperious grandmother form the old country and Ewart James Walters as the tribal leader.

But there’s no real momentum in the show, which veers towards self-parody, with disappointing music and a lot of foot-stomping. Terry Pratchett says he saw his book as scenes in a cinema as he wrote it. I did, too, as I skimmed its pages, but the stage reality remains earthbound and unmoving.