I worked it out, so I thought I’d share in case it helps anybody else.
First of all the jimny is a vvti varient, which has the filler bung on the passenger side. I have a non vvti gearbox as well which has the filler on the drivers side, so I was looking on the wrong side initially.
After finding it the only way I could undo it was by holding the 10mm square bit with vice grips (also needed to put some shallow cuts into the bit for traction for the vice grips.
There might be an easier way to do this, but this is what I did.
https://pasteboard.co/7GOYovZkn1VS.jpghttps://pasteboard.co/acNEiYv72kDL.jpg