Problem is that on nearly every occasion I have to do a search to find out what it is. I've looked through a couple of books, one being the Collins Field Guide for Insects and the only caterpillar that comes close to this one is the Grey Dagger (Acronicta psi); but the colouring isn't right. The illustration shows white below the red spots and grey up to the yellow, whereas this specimen is black; so if anyone can help I'd be greatfull.
When I do identify it properly I'll amend the post.
My thanks to the warden at Pensychnant Conservation Centre and Nature Reserve in North Wales who offered this information
It is a Grey Dagger caterpillar. It is not too surprising that it doesn’t concur exactly with the illustration in your book because caterpillars vary considerably between instars, making identification difficult.Grey Dagger is a common moth but this is an uncommon record because Grey Dagger is virtually indistinguishable from Dark Dagger, so it is usually recorded as Grey Dagger agg. However the caterpillars are not alike so we can be certain that this is the Grey Dagger sensu stricto.