Fitting downpipes tomorrow, they're rotted to the extent that the visible end of the studs are flaking apart. Any tips, and is this a nightmare situation?

 

just be patient with everything, soak all the nuts and bolts in penetrating oil before doing anything. breaking one of the studs off would make it become a nightmare.

 

Can you run the bike before hand? I'd start it, get it warm, and spray the crap out of it with pb blaster. Sonja is right about patience. Don't rush it!

 

Exactly what CorruptFile said. In addition make sure to be even/square on it when you start turning.
Sometimes if something is stuck it can actually help to try and tighten it just a tad. Seems crazy but its worked for me many times

 

Thanks lads, first came off easily and 2nd one in just disintegrated as soon as I tried to put minimal pressure on it, leaving not much more than a stump, off to the mechanic she goes......................

Incidentally, would the blowing down pipes have been a reason for random dying on idle?

 

Yes it could.