It still sounds like the pressure regulator O rings to me. If the regulator O ring splits or doesn't seal tight, then the pumped fuel simply flows back to the tank via the return line. This gives the impression of no or very low rail pressure. It's definitely worthwhile changing the regulator O rings as they cost about £5 and take an hour or so to fit.
Thanks Nodge, took my old pump apart and found the green o ring was ok but the white support collar had seen better days. Picking up the a new collar and o ring from the Bosch agent tomorrow, he said they would be pennies. If that fixes the problem I will not be a happy bunny.
Edit to add:
Do you take the starter off for easier access, I'm gonna take the inlet manifold etc off again as it will make my life easier, but wasn't sure about the starter