Not aware of another relay, but problems with the pump in the tank aren't unknown with the diesels, You can gain access from above by lifting the carpet, theres a panel in the floor you can see once the carpet is lifted up, If you start lifting it back from the rear righthand door you don't need the whole lot up (remove the seats first obviously, and I think you need to remove some of the plastic holders that latch the seats down as well).
If you want an easier way to confirm whats happening, a bit of clear fuel hose to replace a section at the front will show you whats coming up. Not sure if you'd hear the pump running if it had no fuel in it at the time from inside the car. its probabbly worth doing that first as you don't want to go into the tank unless you really have to.
Its also possible you have something in the tank that shouldn't be, its worth a good look around the pump area when you have it open if you go down that route.