Disco 2 Can't get rid of air in fuel pump (TD5)

Our (new to us) 2003 TD5 auto is having a few issues. It had a "cheap" Eurocar Parts fuel pump fitted when bought, and would run for a while, run rough then cut out and be a pig to start. Thinking it was down to the cheap pump, I've just replaced it with a VDO pump - but it still does the same thing ! It sounds like there is air in the pump when you first switch the ignition on (without starting the engine), but it settles down after a few seconds into a quiet hum. Turn it off, then turn it on again and it does the same thing - whines like theres air in it, then settles down. I've tried the auto purge routine (press accelerator 5 times) a number of times, and whilst it goes through the routine OK, the pump still sounds like it has air in it afterwards.
So I replaced the air bleed valve in the filer head but again, no improvement. The filter looks fairly new (Bosch), so I don't think that's at fault. Is there any other way of bleeding the fuel system ? Is this just normal TD5 behaviour ?

I ran through the auto purge routine 5 times in succession this evening, and on the 4th cycle the air was gone from the pump. I ran it through the 5th time and still no air audible in the pump. So I let it sit for 10 mins and turned the ignition on again - still no air in the pump. "Yippee" I thought, and started the engine. I drove it up and down the drive a couple of times and stopped the engine. Turned the ignition on - air in the pump again. SO it appears its only being drawn in when the engine is running. I've ordered up a set of genuine injector washers and seals so will fit those over the weekend - fingers crossed that is the issue.