I am wondering if you are checking your car against the IVA manual ?
In the section on seat belts, there is the following:-
Note 2: A suitable single bolt fixing of adequate strength would be, for example, a bolt of at least 11mm (7/16") diameter of grade 8.8 (the grade may not
be shown on a bolt produced for a seat belt anchorage). Other bolt fixings may be acceptable providing they are of equivalent strength. Two adjacent
seat belts may be secured by one bolt. In this case consideration must be given to the additional loads on the anchorage
Just in case you have not looked at the IVA manual, HERE is a link to the current version (today 2023-10-14) on gov website.