OK, people. Had the fuel pressure tested and the pressure at idle is way too high (3.2bar - it should be 2.3 to 2.7bar). Removing the vacuum pipe from the fuel pressure damper (below Air Flow Metre) has no effect on this. Removing the vacuum pipe from the fuel pressure regulator (same position on the fuel rail on the other side of the engine) increases fuel pressure and replacing it reduces pressure (back to the high setting of 3.2 bar).

The damper is being replaced and then all setting and mixtures will be checked again. More to follow...:)

Pressure regulator replaced. Situation much improved. Once the car is fully up to temperature, no more sub-dipping. Throttle response when pulling away is much cleaner too, and there is slightly less exhaust popping. It still dips while warming up though and is running at about 3% CO when warm so still a little rich. Next step will be to set the mixture and Idle while set up on a gas analyser to see if this can now be dialled down after correcting the idle fuel pressure.....