Bench seat removal

Can someone tell me how to remove the rear bench seat from a saloon model? I'm trying to remove the base part of the seat to replace a broken seat belt. The haynes manual simply says grasp seat at the front lift, slide forward and remove, which obviously doesn't work. It seems that there is a metal catch at each side that the seat is somehow mounted on and is stopping it being removed. Any assistance greatfully received, as its MOT is approaching.

I've googled it and seen other people with the same problem but can't find the answer.
:1zhelp: