[Laird Scooby]

I think you've come up with a diagnosis there Pete, blockage between the fuel filter (possibly in the filter itself, worth changing for the sake of a few quid) and also possibly the pressure valve inside the fuel distributor is stuck. You could try a bit of percussive maintenance with the hammer in the area of the valve on the fuel dizzy, did you add the ATF to the tank?

You could also clamp off the return line to "dead-end" the fuel pressure so you get maximum pressure from the pump through the fuel dizzy and see if that forces out any more rust flakes (nut sheel particles) but don't do this with the injectors in the inlet manifold as you don't want any of those rust flakes getting inside the engine!