either the fuel pump damper seals or pump mate then by the sounds of it!! ur problem is the same as mine was, fuel starvation especially if its not returning to the fuel tank, should literally be pissing out of there back to the tank if its not turning over properly.

Before u go buying a new pump get the tank off and have a look at whats going on, u should see if the damper seals have a split anywhere. Its literally a plastic thing with 2 seals both ends, the seals are part of it so u have to replace the whole damper.
Theres no real easy way to check before taking the tank off unfortunatly!! A quick test before u go buying a new damper is just use a peice of pipe to join the pump to the metal outlet pipe and then trying the car and see if it runs. You may even be able to get away with just a fuel pipe connecting the 2 and no damper?! i was going to chance it but decided to just buy a new pump damper instead!! but dont hold me to that

Least then if it fires up and holds good pressure then u know it was the damper and not the pump or visa versa

Shame u didnt post this a few months back as i scrapped my old 2i with the tank/pump damper etc still in it