the rear wheel bearings are housed in the discs. you can pick up the wheel bearing from gsf. cant remember how much, but its not a lot. just a case of taking the wheel off, taking the caliper/carrier off, removing the disc, change the bearings and then put it back together.
although maybe the hub nut (behind the metal dome thing, centre of the wheel) is loose.....
either way, garage should only charge an hour labour if you buy the parts. not sure what the going rate is... £20/hr?