The exhaust collector-box is pretty damned rusty and by the look of it, has alreay been patched up with welded panels. Basically though, underneath the rust it doesn't seem too bad so I'm gonna see if I can salvage it.
I removed the loose rust from a section and then treated it with Kurust and in all fairness, it's come up pretty well. I could very easily paint over with VHT paint and it would look more than acceptable, BUT, will the Kurust treatment below the VHT paint react with the temperatures? A mate of mine suggested testing it with a blow-torch which sounded like a damned fine idea, so I'll pick one up and try it.
UPDATE : Heated the box up from the inside with a blowtorch and it appeared to cope okay, so I'm gonna go ahead and treat the whole thing with Kurust, then overpaint with Very High Temp paint.