I proceeded to remove the head cover to take a look at the spring and upon removing it it slipped and hit the no5 rocker arm. The valvetrain looked brilliant so I did another pressure test with the head cover removed and could not find any leakage. Replaced the cover and compression was 15bar. I suspect the valve stem might be bent and caused it to get stuck, the mishap of dropping the cover must have loosened it up? Have been driving the car for 3 weeks with brilliant performance. Will keep an aye on it and when it acts up again I know where to start looking at least.