Turnpin: Even though I find that unlikely, I'll measure the O/D and do a search on that, I suppose it's worth a go!
V.I.: I already have a boss, so don't need to add a strap, and in fact I'd have trouble fitting one to what I have now. I've tried City Plumbing who sold me a 50mm S.W. to 50mm pushfit (now have to take it back). Don't really want to try Graham's as every time I ask them anything they don't seem to have a clue, but I suppose I should, just for completeness.
The problem is that many boss pipes have 50mm outlets, so you can just S.W. to them direct to go to 50mm, and I guess that's what folks do, however mine was from Wickes (or possibly Screwfix, can't remember which), with these 63mm outlets and it seems only 63mm x 40mm or 63mm x 32mm adaptors are catered for by most suppliers.
I have found one - just one - company on the interweb called Nationwide Supplies that do them, but they charge a handling charge for orders < £60 making it cost £10 delivered, and I'm worried it might not be quite the right size when I get it.
I can't believe only one company can supply these.
