Retrieved the bike today. Its totalled! Forks are obviously kaput but rear triangle and dropouts are bent beyond repair. Not sure how that happened.
Looks like some internal rusting in the fork steerer had led to a couple of points of weakness that had become stress risers. Bit surprised at the rust as the bike has always had mudguards fitted. Still not completely immune to the elements I suppose. In hindsight I'd noticed some creaking from the front end for a while, especially when honking out of the saddle, but attributed it to a recent wheel rebuild. A couple of years ago I ignored a similar sound and my handlebars snapped - leading to another face-plant - so the moral of the story is: If the bike makes an odd noise and, in particular, its coming from the front end; INVESTIGATE. Do it properly. What looks like an insignificant gouge can actually be pretty significant if it is in a position of stress. I go through this pain so you don't have to ...
Anyway, on the upside, my face is healing nicely and I can look forward to ordering a spangly new touring bike via the cycle to work scheme. Any recommendations?