We had a session to get to the bottom of the problem...... good news or bad.
First thing was to eliminate a valve problem with a compression test. All cylinders were around 60 to 65 psi, happy with that.
Next was to check and adjust the tappets, more a noise problem than being the cause of a sluggish Matilda. They were nearly all out, so adjusted and ticked that of the list. Used this magical Snap On tool that I found on Ebay a few years back, makes setting tappets a dream instead of the normal nightmare.
I was starting to think it was a carby problem...... until I cleaned and checked the points. They were a bit burnt and the gap was about 35 thou. Reset at 23 thou and bingo, Matilda is now behaving herself, running as smooth as a Chev 4 can run.
Who is a happier (and relieved) Chev owner now?