Thanks Ozzie. I need to get myself one of these manuals from somewhere!
After I posted that up, I went on the Comma website, they have a decent product selector and there it states:
Capacity 3.6 litre + oil level sensor
Capacity 3.2 litre service fill
No idea what the oil level sensor business is about, but I will use your 3.6 litre recommendation and go with that.