Fnally managed to machine a fitting to pressurise the cylinder. I turned the crank until the intake valves opened and then closed and then another bit to ensure the piston is on the compression stroke and all valves should be closed. I then pressurised the cylinder at 50kPa and could hear the air flowing through the engine and found it blowing out the exhaust. I then closed the cock on the compressor and increased pressure to 500kPa. Opening the cock drains my 24L compressor through the exhaust in about 30s. The exhaust valve is definately stuck open in this particular cylinder most probably due to a broken spring on the valvetrain.

I managed to look at the piston with an endoscope and could not see any damage due to the piston hitting the valve. Has anybody replaced one of these springs? Can it be done without removing the head or camshaft?