Hi MartyB, Yes the engine seemed to be misfiring / lumpy and I think now we are certain that it is overfuelling and burning in the exhaust manifold (hence melted cat) and also getting into the sump (hence petrol in oil) as people have suggested. The garage has tested the oxygen sensors (and many other things) which seems to be in spec but after further inspection, it doesn't really seem to be responding / changing its readings as the engine warms up and hence I assume the ECU will be telling the injectors to pump more/max fuel? If the root cause does turn out to be this sensor, my question to