Updating an old thread, but this is finally sorted. Turned out that 5 exhaust manifold gasket studs had snapped, both the ones on cylinder 6 and 3 at the front as well.
My local place wanted to take the head off and drill them out, so I told them to get stuffed and took it to a place up in the valleys, a place where the owner's project car was a 400bhp SR20 S13. Got them drilled out with a right angle drill, new manifold/turbo/exhaust gaskets fitted, it sounds lovely now.
Now I just need to fix a grounding issue that has developed, and a sticky solenoid in the starter motor, pretty easy stuff by comparison.