Well, I finally sorted it at the weekend.
I did think about drilling out, but the on,y bit long enough was a masonry drill bit -- thought that might make too much of a mess!
In the end, a good few wallops on the underside of the SFN with a socket wrench extender pushed the top cap far enough out to hacksaw the bolt. Saved the nice Onza top cap too!
Bashing out the SFN in the conventional way did indeed reveal two SFNs (lazy bugger who sold me the forks!). Looks like the bolt screwed into both SFNs, somehow causing the top one to fail at the interface between the threaded centre and the star fangled part. Oddly the bottom SFN seemed in better nick than the top one.
Anyway, new SFN tapped in, a new spacer (wrecked the old one), and it's all good.
The Boy may have learnt a few new adult words along the way though!