Fixed
Cause? Feckin vacuum pipe to manifold
I have already checked this a few times, but nothing to see until yesterday I decided to remove it (for another reason) and it fell apart in my hand!
The split was behind where I was looking so I couldn't see it.
Anyway, I cut off the split section and used heat shrink tubing to fix a section of (wait for it) air line hose to join up the short section. Lighter shrunk it tight and towed a car 23 miles north today and WOW!
Soon, I am mostly purchasing new rubber (or better) pipework for all the stuff that will need done.
Really REALLY pleased though