My money is on the rocker cover gasket... the rocker cover has 13 bolts which sit in metal top hats which have rubber grommmets under them, it's these that provide the clamping pressure of the cover onto the gasket.
Replace the grommets and the gasket and the leak should stop.
There's an acoustic cover that sits behind the head, if you remove this (2 awkward to get to bolts at the back of the head), you can use a little mirror to check for oil weeping out of the gasket - it's most common for it to leak out the bottom of a black half moon bit which seals the back of the rocker shaft... clean off the back of the head and for a drive first before checking.