Thanks for all the replies folks, just a bit sad how many of you were sitting in front of a computer on a Saturday night!
Couldn't find any bolt croppers in Halfords. The chap in the cycle department sold me some cable cutters which didn't do the job -and that was after I had to explain to him exactly what mudguard stays were
Also bought another mini hacksaw which turned out to be just as blunt as my old one, probably a H&S directive.
The best tip was clamp the stays in a vice to cut them.
Overall the job took a fraction less than 6 hours, much of it spent crawling around the garage looking for the bits I'd just dropped. And it wan't even my bike. I don't think I'll open a bike repair shop any time soon.
--
Bryn