Hiya,
Just adding my bit.
Halfords do a high temperature copper silicon sealant,
LOCTITE 5920. I think it's around £6 but it works well
on any joints especially the exhausts. Just smear it inside the
head and push the pipes in.
I use this stuff on both my Classic race bikes ('67 T20 Suzuki and TZ350 Yam). The pipes on those are loose (held on with springs and a rubber mount) and get heck of a battering. It works a treat. Well worth the cost.
I'm having similar problems with badly fitting pipes on my A10 (small part of siamese too narrow and so comes loose) and after trying other solutions like welding a band around it, I'll be using this stuff after I've put the rocker cover back on. Normal silicon sealer is pretty good too though on most things and worth a try.