One of my rear bumper mounts has rusted through so it's dangling on one side just rear of the wheel arch...

1 Will it fail my MoT test on the 4th Jan?
2 Fix or replace? I know a welder/fabricator...

I would get it fixed/replaced if you know someone that way you know it shouldn't fail on it

Those mounts just support the ends so you can even use a tie wrap if you want. Doubtful it will be an MoT fail.

Cool. Cheers. Getting in there to drill for cable ties or rivnuts is tricky, I'll try to get some welding clamps in there temporarily.