I'll give it a go when the rain clears up, failing that what are the other possibilities? I was planning to rebuild the engine anyway, although I was hoping to do it in the summer...

Makes a lovely big cloud on startup, and when under strain. Oil should be fine, it does have a minor leak however

https://www.youtube.com/watch?v=eBDm9vli2IY Not the best video, but still. A mechanic recommended replacing the water pump which I have done, but it made no difference.

My 1991 200tdi has some pretty bad engine knock, but only when it's warmed up; sounds fine when cold. As the revs come down it lessens off a bit too. I'm thinking it could be the timings out? Oil pressure gets low when warmed up and idling too, although that happened before the knocking...