Is there no way you can take these pumps to a diesel service centre and get them reconed.. I know you have mentioned they are sealed units, but I would take that with a pinch of salt from a Vx dealer.
Keep all options going matey.
Annother daft thing.. I am going back 32 years with this in mind, so may be clutching at strws. My 1st job from school was recondotioning pumps and injecters and training as a diesel engineer (had to leave coz of dermatitus after a few weeks) but most of the problems with one bunch of vechicles was condensation in the fuel. Now despite all the filters and they could just not start thier units.
Now modern cars have moved on a hell of a lot I understand that, and I admit I know knothing about the new technology. But with the condensation thing all those years ago it's the one thing that has put me off diesels.
All I am thinking is could dirt/moisture be the simple problem. Dealers dont do things the old fashioned way and take it off and clen it, they just keep replacing parts nd expect the customer to stump up the bill.
As I say, Just a thought.