I had a fuel (diesel) pressure testing rig made up at a local hydraulic hose shop - screws into where the temp sensor is (fuel pressure regulator) on the driver's side of the engine. My fuel pump was original (2003 Td5 Disco) and I found out that the pressure was virtually zero when the fuel pump was on and the pressure tester screwed in. There was 12V to the pump, but somewhere inside the pump (the HP stage I'm presuming) there was something not quite right; effectively the diesel was almost being "sucked through" to the engine. Purchased and installed a new fuel pump (and fuel filter) and all good. If you still have the original fuel pump installed then it is likely that if it isn't already playing-up then it will soon (16 - 17 years is a good innings). Can't think how your observation fits in with anything else at the moment.