Bit more info attached. Looks like that once the relief valve "relieves" it is pooped and has to be replaced. At least thats how I read it. Have you got a workshop manual? pages attached in case you don't have one.I've gone through the manual as well, the Pressure release valve on the HP rail could be an issue but 2 things have stopped me moving and replacing it, one being if it is faulty then fuel pressure would be affected no matter what the temperature of the engine is and secondly, it throws up an engine management light if it goes into fault I believe? Or it should show up on the OBD reader as a fault and yet I get none.
Bit more info attached. Looks like that once the relief valve "relieves" it is pooped and has to be replaced. At least thats how I read it. Have you got a workshop manual? pages attached in case you don't have one.