Hi Pete,
the two main systems are Bosch and Delphi, Delphi use IMV and bosch use pressure regulating valve or propotioning valve. Delphi control inlet at the pump as you have described, Delphi also use two injectors to control rail pressure. Bosch control inlet, outlet or both. The proprtioning valve, on the pump can be set as open or closed with no supply. 700-900bar as you already know is very low for accelerating, have you checked pressure signal at PCM, either scope or current data, does anything else disappear i.e. cam, crank, injector signal when the vehicle stalls? Is the return line free of damage, kinks etc? Was the pump timed up when fitted, were the injectors coded into the vehicle. If older bosch system, X and Y injectors, all Y types preferred.
HTH Jim