Hi
I am afraid we have a miscommunication.
I thought you were talking about the upper bolts in between the headrests. Where the belt slides down in the retractors.
That wrench is 12mm. And try not to use an open end but always - when possible - a close end and the best is 6 sides instead of 12 sides.
Make a picture so we know were you are working.
Thank you very much indeed for the closed end 6 sided tip - it worked. One bolt is completely out and the other half out. I am so relieved but will need a couple of new bolts. Now the search is on for suitable belts.