Last week I saw another drop of oil and a quick check showed the leak appeared to be coming from the same place. However closer inspection showed the problem was that the right angle pipe had come away from the orange valve on the breather plate. As I re-attached the pipe to the orange valve I realised the pipe is 15mm too short and is essentially being pulled away from the orange valve. Given that the pipe carries oil vapour and a bit of oil to the air intake then it gets well lubricated and before you know (well in about a year) the pipe has come away from the orange valve.
I cut a new pipe today and the extra length means the pipe is essentially pushing itself onto the orange valve. Time will tell if this works.
This is the old pipe. The new one is just 15mm longer.
The old pipe had come away from the orange valve.