Anyone,

How can I get to the centre seat belt mechanism as the belt will not release, without ruining the seat/

Is the seat properly locked upright, the seatbelt will lock if it isn't.

Failing that you need to take the seat back out of the car and carefully remove the cover then the foam. You will be able to do it without removing the headrest socket things and seat release handle but obviously you can't remove the cover completely.

Assuming it is the same as the octavia, lie the seat face down and with a screwdriver and pair of pliers pull the edge of the seat cover out of the groove it sits in at the sides and base of the seat back. Just take you time and be careful and you should be OK. You may find though that once you have the cover and foam off some of the mechanism is riveted to the seat back and can't be removed easily.