NOEL COWARD, who knew a thing or two about musicals, apparently departed Lionel Bart's lavishly staged Blitz announcing he had enjoyed ''humming the sets''. After seeing this exquisitely staged Royal Shakespeare Theatre Company revival of a 10-year-old, award-winning Broadway show, I know just what he meant. Antony Ward's sets, all translucent grey panels on which are traced the skeletal shapes of trees, which slide hither and thither to reveal vistas of misty parkland or grandiose big house, are gorgeous and atmospheric.

The show, stunningly staged by Adrian Noble, enjoyed a success last year at Stratford as the company's Christmas show. Little Home Counties' girls will still enjoy this account of Frances Hodgson Burnett's novel about stuck-up orphan, Mary Lennox, sent to Yorkshire to live with rich Uncle Archibald (Philip Quast in fine voice), a widowed hunchback, grieving for his wife, with a bedridden son, and Uncle Neville (Peter Polycarpou), a doctor with malice in mind. There she discovers her dead aunt's ''lost'' garden, helps her bedridden cousin, Colin, to walk, and is befriended by assorted yokels and maids who dance merrily around the place as she brings the garden back to life.

Marsha Norman's book and lyrics are undistinguished, Lucy Simon's music second-hand - the big aria opening Act Two, Lily's Eyes, is a dead ringer for In Buddy's Eyes from Follies.

But the show has been strongly cast. Linzi Hateley makes a spirited nursemaid, Martha; Craig Purnell sings his heart out as her nature-loving brother, Dickon; the divine Dilys Laye is the horrid housekeeper, Mrs Medlock, to perfection; and Freddie Davis amuses as the avuncular head gardener, Ben.

It is, however, a secret I do not wish to share ever again.