Sorry Pete for the delayed response I must remember to tick notify me by email! The vehicle has a pre cat oxygen sensor reading of 1.2v flat when at operating temperature. This reading can be manipulated with induced air leak. Emission check shows very rich vehicle until the fuel trims pull it down. I initially found 2 cylinders failing a cylinder leakage test and removal of the head proved the exhaust valve seats no longer sealing. This has been repaired. I have now removed the spark plugs and with ignition on checked with emission meter into the cylinders the hc's. it appears cylinder 4 has always got around 200ppm after a rotation of the engine (checking the other cylinders) and the other cylinders are never higher than 25ppm. Would one injector leaking under pressure cause an excessive rich mixture??