Are both studs on the rear cylinder head supposed to have a spacer bush, or just one?
Mine has only got one.
Any tips for removing the exhaust system? Mine needs a good clean/overhaul, I believe there is a crack in the rear header - can this be welded?

There is only meant to be one. It's a weird set up - there is meant to be one on the front too - I don't even know what its purpose is. Maybe just to help make sure the clamp is more accurately sitting at 90 degrees to the exhaust pipe?

There is a bolt at the rear, (not exhaust stud nut - one of the connecting ones), that is awkward to get to and if it's corroded up it's a bitch. Soak all of them well the night before with penetrating oil or WD40 and disassembly will be easier. Also watch out for the clamp nuts if they look corroded because when I did mine one of the front studs snapped off instead - had to use a stud extractor to get it out and replace with new one. I would suggest that maybe it's wise to replace all four while you're at it. Gear4 does next day delivery and they are pretty cheap.

Thanks Bic - that's really helpful. There is a lot of corrosion generally - I will approach with caution!

The crack in the header can be welded. I just did one for a buddy of mine. Just make sure you purge the tube with the inert gas before welding, or you will get carbon burnthrough on the inside. If the welder is good, it will look like it was welded from both sides...usually called a 100% weld.