To get on top of the camshaft/valve cover the oil is either coming out of the oil filler opening due to the cap not sealing properly or is squirting up from below or being blown up by the cooling fan. Otherwise if the oil is running down the sides of the camshaft cover its seal may be leaking. Try cleaning away as much oil as possible from the engine or get it professionally cleaned and then carefully look for the leak with the engine running. Tip - use a good torch in the dark to look for leaks that would be hard to see in the shadows under the bonnet cast by strong sunlight. Correction - looks like you're in the UK - the shadows cast by the weak sunlight.