When you take off the cam cover, clean off the bottom where the gasket goes and check it with a straight edge. I suspect it may be slightly warped which can be caused by uneven or over tightening when it was put on. If it is take it to a machine shop and they can mill it down so its perfectly flat again. Fit a new gasket and tighten it down in the correct order with a torque wrench and you should have no more oil leak. Good luck.
Cheers Richard from Southampton.