Perhaps it is exhaustion, therefore, as much as solidarity with the spirit of St Trinian's, that explains why, on the way to the set, Rupert eats three Cadbury's Creme Eggs in between puffing a small roll-up.
'I wouldn't normally do this,' he says, inhaling chocolate and tobacco with equal alacrity, 'but I have to get into my schoolgirl frame of mind.' Then he flicks through the pages of Hello!, giving an acerbic commentary on the celebrities featured within. 'Look at that, another ghastly evening at the Met. Forget St Trinian's, I should really be the headmistress of a star academy. All these so-called style divas have such terrible dress sense.'
His voice becomes increasingly disapproving, his dark eyebrows raised ever higher, as he regards an assembly of female stars at a New York ball. 'I could give them a proper going over, and then put them back on the road. Ooh, look, Kate Moss's hands have become almost as grabby as Madonna's. Actually, they're all at it, clutching their handbags with knobbly claws, like it's stuffed full of cash. That will have to be one of the first week's lessons at school. How to hold your handbag without looking grasping.'
This is followed by a meandering discussion about snogging Colin Firth.