If you are getting oil on the plugs, it's probably time to do the rocker cover gasket and spark plug gaskets. They come as a set and are about £35 from BMW 11120030496. It's only one engine layer below the plugs so shouldn't take too long to do. While you are there take some pictures of the cam chain and its tensioner. The extension on the tensioner hydraulic ram will indicate how stretched the chain is and if it needs replacing soon.