I've diecided to try the easy way first.Bolted everything together with 7 studs only. Had to jam a 8mm nut between pipe and bracket where there was only one stud to stop the bracket moving around. It worked. No smoke coming out form between head and the pipe. I'll have to check properly when I go to work next time, but I think it's fine.I didn't know this thing could be so quiet. I had 6cm long hole in the 4 to 1 collector thingie under he engine. That has now been welded and it's so quiet you can talk next to it without shouting .
instead of carefully cleaning off any road dust or raindropspainstakingly take out all the exhaust studs and replace with stainless steel bolts exactly the right length, using copper grease.