“Don’t ask too many questions – just go with it,” suggests a programme note, and that was certainly the mood abroad in an indulgent first night audience for a fairly broken-backed and relentlessly camp new musical by Ian MacFarlane.

McFarlane, who also directs, can obviously write adequate generic musical theatre songs, and some of his lyrics have a pleasing intricacy, but his story of a blocked Manhattan novelist being dragged by his new gay room-mate into a parallel universe of curses, spite and sexual sadism in an enchanted forest is less than compelling.

It’s one of those shows that seems more like an excuse for a bad musical than an argument for a good one. And even the surprise presence in the cast of Ellen Greene, still best known here as Audrey in the original Little Shop of Horrors, fails to galvanise the proceedings beyond the tawdry, Audrey.

Greene comes on strong as a nymph queen and an enchantress, melding both roles in a succulent seduction turn in a pair of tight shorts that has the lost writer Bailey Howard (an attractively geeky Benedict Salter) admiring their contents and his new out-of-work actor friend, Cooper Fitzgerald (an amusingly narcissistic Steven Webb), admiring their stitching.

Yes, folks, Bailey and Coop go through a Narnia-style door to find themselves caught up in a bonkers scenario in (witch) which a medieval prince has gone missing as a day-time soap star and deluded actor manager called Garbo (Peter Duncan has a high old time as all three), and Bailey falls for the disembodied head of the cursed and boxed-in Miranda (Ashleigh Gray), more cooped up and physically challenged even than Nigel Harman in Shrek.

Their love duet has a pleasing grisliness, and Greene’s torch song, which she kindles in her trademark lisping huskiness, has the right sort of tacky glamour until it outstays its welcome. There’s a soupy number about, guess what, daring to dream, and a few lively ensemble sambas, rags and tap-dancing routines. George Dyer leads an onstage trio from the keyboard in odd socks (one pink, one green).

The camp highlight is the tango duet in which Miranda declares, only too accurately, “I’ve got no body but you,” but aficionados of such things will also relish a quartet of inexplicable Mexicans, Peter Duncan looking for his light as Garbo up a ladder, and Ellen Greene shaking her booty like a young Fenella Fielding at a Roman orgy.