Bus bee you're right it's the one on the motoworks site. I may give them a cheeky call and ask what size it is. The tools are available I see just need to nail the size
I have this type of filler cap fitted to my bike. Although it looks like a security pin torx, it isn't. Normal torx have six lobes, this has five lobes. It may be the case that only BMW can supply the key.
Alan R
A tamper-resistant version of Torx Plus exists having five lobes rather than six, plus a solid post in the center, and is used for security as the drivers are uncommon.
Hth