[PinkPig]

it was something really technical and very expensive.

Yep - a blocked vacuum line that goes off to some sensor or actuator somewhere apparently. Garage sorted it, now it works, I hope for a while....I'll let you know. it was slowly getting worse, heat causing the pipe to expand / contract and eventually running only on enrichment (choke)

Thanks to all for all the suggestions:-)

PS_ flywheel sensor is on top of the clutch housing - right hand side of engine as you look under the bonnet