Evening All,

Thanks Paul for your input. Tried what you said but it had no chance of working because I missed an obvious problem with the car for which I have admonished myself with a kick up the back-side. Although the Crank Sensor trace looked ok when pico-scoped, the bottom pulley had a tooth broken just prior to the "pick-up" gap and this caused some sort of mis-triggering.
Never noticed it initially but closer inspection found the fault. Got there in the end.

Regards

Pete M