Hi Eamonn

These engines ( if it is an N42 Engine code ) are common for the valve timing jumping due to the chain tensioner getting weak.If you have spark and fuel pressure and injection may be worth checking the valve timing.
Special tools are required to check and adjust it but if you dont have them and want to confirm the theory remove the rocker cover (easy enough) , get number one piston on tdc then check the flat edjes on the cams at the rear of the engine should both be flat and inline if not the timing is out also check if the plastic timing chain slipper has broken up if the timing has jumped.

Hope this is of use.

Steve