Is the breather on the oil filler connected to anything else?
It should be connected to the air intake. If its not, then you must reconnect it. Mine used to be dangling, used oil, then I reconected it, but the hose collapsed/kinked. this made the leak worse. I have since replaced the cap (thanks wingnut) and used a new hose. its a lot better.
The hose needs to be connected to the sir intake as this sucks the air out of the rocker cover. if not the pressure inside builds up, and blows the seals.
Is the rocker cover covered in oil? You can refurb the seal on the filler cap for about £2. Also change the rocker cover gasket, about £2.
Worst case, and I think whats kinda happended to mine, is that the rear crankshaft oil seal leaks, and needs replacing. This is a gearbox/engine out job.