As I recently posted, I'd been trying to trace the source of the fumes, whilst corking up all the various holes through the bulkhead. Various knackered grommets replaced, and replaced the seal on my fibreglass gear tunnel-the bottom flange is about as flat as my arse! Only after peering at the exhaust manifold closely did I spot the hairline cracks, now replaced with a serviceable spare. Interestingly, since replacing the manifold, a noisy 'tappet' noise has now disappeared. I'd checked the tappet gaps several times, but could never pin it down. It would gradually quieten down as the engine warmed up - probably as the cracks closed up?