After some deducing, and lots of listening, this is the conclusion...
I've been advised to have the heads tightened.
Now, as you all know, my engine knowledge is dangerous at best at present - yes Fig, I am learning!

Is this something that is do-able without a complete rebuild, or should I just wait and have the engne rebuilt at the same time?
The engine is sitting on about 103,000 km - genuine.
It has never been rebuilt or rebored (spelling?).
N.