The engine does not stall, there is just no response from the throttle until the EML goes off.
My understanding is that the actuator on the bottom of the pump is a timing control device.
I have found
this pdf which is about the vw tdi engine, but the omega injection pump is just a 6 cylinder version of the tdi one.
If the timing control actuator was faulty surely the fault code would be more likely to be 34 - injector timing out of range?
The description of code 64 is fuel quantity control out of range, which to me suggests the fault is with the quantity control position sensor, (G149 modulating piston movement sensor in the linked pdf), which is just under the top cover of the pump.
All it will cost me to have a look is a top cover gasket so I will get round to it at some point.