The culprit: a leaking thermostat cover. Unusual, but not unheard of - the forums have a few reports of the cover cracking - and fortunately not only relatively cheap to fix but they even had the part in stock.
Of course it's never that simple, and while the car was up on the lift they spotted that the exhaust was blowing (didn't I go through that last year with LOUD MODE). It turns out that while Halfords did replace the exhaust, they didn't do a good job of it as the joints are blowing. So at some point I need to trundle round and politely rant at them.
Ah well, it's all part of the fun and games of owning an Alfa.