You were on the money. Number 1 cylinder bolts for the exhaust manifold where almost completely out (any longer and I probably would have lost them). Number 2 were loose too.
Being a public holiday so I couldn't get a new exhaust manifold gasket I just retorqued them to 25nm specified - Rave didn't say anything about using Loctite or anything on them, but I'm wondering if that might be a good idea now.
Noise has gone, but I do want to replace the gasket at some point now.

