I doubt it would be picked up on the MOT as they don't go fast enough on the brake test for it to show up. I has the same issue on my focus after changing the front and rear wheels over. I got them re-balanced for a few pounds at a local tyre fitters and the problem was solved. Worth it if you can see there are no weights on there and is the cheapest first port of call IMO.
Cheers pal, need a couple of new tyres so will get it done then