Hello again.

Thanks for your replies. Turns out it was the head gasket

Getting the head off has proved to be trying - half the bolts on the cylinder cover and inlet manifold were so rotten, we ended up having to hammer sockets on them three sizes smaller. One bolt under the inlet manifold refuses to budge, so we've had to force the manifold away from the head.

I've decided to replace the camshaft whilst I'm there. Can anyone recommend somewhere to buy one? There doesn't seem to be much about on the internet (or I'm not looking in the right places).

Thanks again.