Hopefully someone else will learn from my mistake. After doing a valve clearance check a mate of mine noticed a puff of smoke coming off my header when I blipped the throttle. After investigating it we realised that oil was coming out of a small hole in the cylinder near the exhaust header and some of that was spitting onto the header. I took a quick video:
https://www.youtube.com/watch?v=gSd0gbL ... YMH_kVNZkw
Turns out there is a seal between the cylinder head cover and the spark plug hole and mine must have fallen out when I did the valve clearance check. Oil is leaking through the gap, down to the spark plug, and then out of the drain hole which exits near the header. I've highlighted the relevant seal in red (4) below:
So when you do take the cover off to check your valve clearances be careful to make sure you have that seal in place when you put the cover back on.