Thanks for looking. It's not external notches, it's an internal pattern that looks like the tool would be pretty much the same size as a regular splined removal tool but instead of having many small splines it would just have 6 large square shaped 'splines'.
I've also spent hours looking at pretty much every tool stockist on the net but no one has anything like it. I will phone the bike company and hope they have some tech person who's been there for 20 years and can remember.
I already tried to loosen it in a mildly destructive manner but it's not budging easily!