Having failed the MOT on the backing plates not being adequate.
At the moment they are just tacked at the corner, I plan to replace them with new 65mm square ones and weld the whole edges.

But I was wondering if people had backed them up with a plate, or if they just had the 55/65mm square plates under the floor pan.

Ken,

The plates should be beneath the car that way acting as a strengthing to the floor pan rather like using a penny washer to spread the load. When we were racing back in NZ our plates had to be 63mm x 63mm 3mm thick and had to have a radius on all four corners. They dont have to be welded but can be if you feel the urge. You should also you a Nylock nut on each eyebolt.
Check the FIA standards as these will be the most extreme in which you should have to comply with.

Its FIA standard plates I have ordered. 65mm square, 3mm thick.

I think the old plates are only 45mm square, so hopefully that will keep them happy once changed.