OK well with everything checked and verified, I took a deep breath and went for tweaking the EHA adjustment. As I understand, this varies the chamber pressure (lower pressure is supposed to be 0.4bar lower?). I wasn't measuring the chamber pressures so it was a bit hit-and-miss.

It's currently set to be richer than initially. This has improved the engine response greatly. However it's still not perfect with a very slight delay and without a crisp throttle response.

Russell

Well I was probably over-eager to judge there (and maybe again now!). Went richer again and problem got worse. Right I thought, I'm sick of this, let's go wild.

So the EHA is now adjusted it hugely CCW (about 3/8 turn) - from what I read this means leaner.
And what do you know, the low down torque is massively improved and I am certain of this. It doesn't have the same throttle lag/delay either but can occasionally mildly hesitate.

Further adjustment required. I can't work out why it's better this way? Perhaps my fuel pressure regulator is actually the problem.