Many thanks for your responses.
Steve, Knud and Andrew suggested using either a 19" or 21". Well, it sounds OK, but the reason why I posted in the first place was because I know of three people who had just this problem. One of them even tried one after the other and had a problem with both...
Adrie suggested I cut a section out of a 21 and re-sealed it. I couldn't live with that! In my motorcycling life (41+years) I have had a front wheel blow out once (Tiger Cub, M6, 1971, eek, help!) and two rear punctures caused by patches coming unstuck. The last one was on the VB going home from the Annual Rally in 2008. When I looked back through my service history I had repaired that tube on my way to work one mornin ten years earlier. Normally I take the view that a repair is a temporary solution. I must have forgotten, and I'd even replaced the tyre since without noticing the patch. Must be going loopy!
I've asked the Vincent community and got pretty much the same responses. But thanks for your replies guys.
Knud, I'll see you in Denmark 2013.
Cheers, m' dears.
(AOMCC Exec Committee & Selly Oak branch rep)
'47VB, '51KH, '58FH, '05 BMW oilhead boxer.