Playtime for newly-arrived schoolteacher Sheba Hart (Blanchett) is a furtive romp in the art room with an obliging under-age pupil.

For the flighty Sheba, the sneaky bumping uglies session is a release from her marriage to an older man (Bill Nighy being, well, Bill Nighy) and years tending her Down's Syndrome son.

However, to long-serving history teacher, the bitter Barbara (Dench), it's the unfeeling betrayal of a fledgling friendship...and one for which shagging Sheba will have to pay.

Director Richard Eyre and screenwriter Patrick Marber may have slightly lightened the tone of Zoe Heller's compellingly bleak novel...but it still packs a heck of an emotional punch.

Barbara Covett is a splendidly creepy creation. A cynically acerbic time-server, who rules the decaying inner-London sink school with a rod of iron, you don't really appreciate the levels of obsession to which she will sink.

Blanchett imbues the hippy-drippy Sheba with just the right lack of caution to embark on a suicidal fling with a spotty teen who wins her affection merely with a slight compliment.

Soon they're frantically coupling - aptly enough - in a railway shunting yard but the feral attraction between the two leads to carelessness...and the attentions of battle-axe Babs.

There's plenty of grisly viewing here - Barbara's cruelly witty observations of Sheba's bo-ho lifestyle (her engaging Downs victim son is dismissed as a "court jester") and few would fail to flinch when Babs threatens to reveal all during a tense stand-off in the street.

Yet things rather fall apart in the third act, the tone swings from comic farce to dark dread and the strong narrative thread begins to fray.

Still, it's never less than watchable and Dench's turn as the emotional limpet, sucking the life blood from her giddy prey will long linger in the memory.

Tim Evans