My Car failed the mot on the Drivers front wheel bearing as it has some play.

What size is the nut?

How easy is it to fit a new one?

Not sure of the size, but I think the hub nut is specific to the car due to the deformable flanged edge. Go on www.shop4parts.co.uk and enter part number S4P1213 in the search.

Actually though, the bearing comes with a hub nut as part of kit S4P619, or S4P620 for ABS models.

its not too hard, i think the nut is a 32mm, it needs replacing. as for the bearing you should be able to get a non genuine one for around £15. if you take the hub off and take to a garage they will press out the old and press in the new bearing and charge you about £5 then you just need to put it back together.