Stupid question... pretty sure I’ve been sent the wrong item from ebay. However any idea what size spline bit fits into the tracks under the front seats? I think it should be an m8

I can't remember, I have one, I think it's the same size as the bolts holding the hatchback hinges if you want one that's easily visible to check.

Unless I’m being really dumb... I really don’t think this is a m8 spline... photo shows m12 spline underneath, with what’s meant to be an m8 on top...

 

possibly the worst photo ever, the m12 didn’t fit, so got an m10 instead, that was still too big, so purchased an “m8” that’s far too small.

 

no markings on the spline either 

M10 fits ours.

the “m8” I’ve posted a photo of above does fit into the boot lid hinges. 

 

It could also be a problem with a supposed m10 spline I bought 

Do you have a Vernier caliper or micrometer?

Across points I'm measuring 9.46mm on the M10 bit I've got.

Should look like this:

M10 multi spline like wino said.