Thanks.
I'm thinking (hoping) age. Car has nearly 187,000 miles up, 70,000 of those under my arse, and this belt hasn't been replaced in my ownership. Timing belt done last summer, but at the time I couldn't afford getting the aux belt done as well. I'd had the car parked & idling for a while yesterday (hot sunny day), so I'm guessing that was the final straw for an old belt.
The car drove home from where the belt failed - 1.5 miles or so. No power steering obviously, but otherwise it drove as normal, so hopefully there's nothing else serious wrong. For now...
EDIT: From the pic posted above, I'm taking it that there's no special tools needed for this job - just use a spanner to relax the tensioner, and fit the belt?
I took my failed belt to the parts shop this morning. Of the two listed on their system for my reg number, one was 1685mm long, the other 1795mm. Presumably the shorter one is for non-AC cars? The guy in the shop measured my old belt at 1785mm, but was pretty sure the 1795mm was the right belt.