Grump.
Many thanks for that, especially the cushion bit. I can see where you're going and I plead stupidity on the grounds of, er? well, pure stupidity I suppose! As you can guess, Corbin didn't supply a bolt or instructions and being more of a consumer than a mechanic, my lateral thinking is somewhat constrained (buying too many Honda parts will do this to you

).
So, if I can get my hands on a bolt with the right size thread on, this will get the solo seat sorted. Only problem will be fixing the front of the Triumph pad as this has a bracket that slides over the post and pulls back before screwing tight at the back of the pad. I'll get there in the end. Biggest problem at the moment is looking at the seat sitting in the spare room, it looks really nice. Oh, what sweet torture we bring upon ourselves.....