For those following the saga, here's a quick update.
Turns out the head has been cracked for a while, once I started digging in I found some traces of old clear silsatic so someone has previously tried to seal the leak. I've owned the bike about two years and there has always been a tiny wet patch there but I didn't think much of it as I assumed it was the rocker cover gasket starting to leak.
Oil starts to visibly ooze out of the crack after about 1 minute of running, it looks like it is pressure feed but I'm still not sure. I can't see any oil galleries anywhere near the leak so figure it is either a very long crack that has traveled very far into a gallery and is being pressure feed, or the crack is opening up as it grows and it is actually a large gap which is letting oil which accumulates in the head flow out at a decent rate so it looks pressure feed.
The plan is to remove the head and work out exactly what I'm dealing with here. There is a decent head shop in town so I will see if it is repairable, I imagine it will be but also have concerns that I'm seeing the tip of the iceberg here and that the crack is enormous. If it can't be fixed the plan is to panic, and then probably source a replacement head if possible. So if anyone has a lead on a second hand head I'd love to hear from you.