Damn. There was a part missing. That's me f*cked.
It is quite simple. The buckle pushes a piece of plastic down the side and out of the way and the L shaped piece of metal springs into place and a little metal bar rolls up to lock the L shaped piece in place. When you release a second piece of plastic pushes the metal roller back down and the L shaped piece pops back out and the buckle is released.
This would be the binnacle (that appears to be how it is described) that is in the 2 rear seats in the middle. As far as I can see it is a seat apart job to get at the damn bolt to get it off.
RR.net says same parts were fitted to Pug 405 in the 90s, some Passats and various other cars.