[dougal]

The Diesel fuel injection pump was what made Diesel-engined vehicles possible. Previously Diesel engines required a supply of compressed air to "atomise" the fuel, whether coal dust, milk powder, veg oil or petrochemical fossil fuel.

But the injection pump (and hence Diesel vehicle) wasn't invented until 20 years after the death of Diesel himself. This fact alone calls into question all those statements about Diesel inventing something specifically to give people *transport* based on pure veg oil.
However that rant can wait.

The injection pump has to deliver an appropriate quantity of fuel at just the right moment (relative to the piston's stroke) at *very* high pressure.
Electronic types make all the quantity/timing adjustments by software, rather than the traditional mechanical methods. (Think of the adjustments in a petrol engine's ignition and carburation systems...)
But it would seem that the common problem with the Bosch VP44 used in some (all?) 2 litre Vauxhalls at around that time is in the part that replaces a conventional fuel pump - the "lift" pump, that tries to suck fuel from the tank into the high pressure precision stuff.
Which is why adding a conventional pump, to assist the lift pump, is a possible cheapish diy fix - but obviously only if the problem is with the lift pump within the "injection pump" assembly.
Hence the *need* for an accurate, specialist diagnosis.