Well, I've pre-emptively been doing what malcolm has suggested. I took the BOV off....
As you can see, my leaking seals have caused some oil to blow out the valve, no doubt causing bits of crap to stick to the oil mist and make the smooth operation of the valve a bit sticky.
So, I stripped it down and cleaned everything. I also noted that the nipple for the small vacuum pipe was loose enough for me to unscrew with my fingers, this may have caused some slight leaking.
Re-assembled, nice and clean (how can these things cost so much money, there's sod all in them)
Here's a video clip of it running having been stripped and cleaned. You can see how low the revs are now that make it vent compared to the other video