Looked at a USA site but they warn of £25 import duties. Anything second hand on ebay looks like junk, the problem was with my CB550f horn/passlamp button which was floppy and causing problems, the CB750 one gets rid of the passlamp bit. In the end, I've re-made the 550 switch without the pass bit, works OK now so I can retain the original switch .
Roll on summer.