Update:
a colleague looking through our seminar room bookings stumbled across two teaching sessions I'm booked to do tomorrow.

The good news is that she found it today and not, say, on Friday. (Plus, knowing I'd lost my diary kindly let me know.)
The bad news is that this is exactly the kind of thing that's hard to track down - particularly if it's over in the faculty where I can't check room bookings like that!

Oh dear, oh dear, oh dear...