1997 Jaguar XJ6 wont start on key but will on a bump start

hi there i have just brought a xj sport manual one on a p reg the car wont fire up and run with the key but will start with a bump start the pressure is up to the fuel rail i can se it when crack the nuts off
i put a gauge on the fuel pressure regulator turned ingintion on and the gauge went off the scale being 80 psi with this gauge connectrted he car started on the key took the gauge off put it back on return pipe it carryed on starting on the key then tryed it again and now just spins over wont fire again whats happening with this car please help its really confusing me