I would have thought that if the OSF brake was not working properly then the car would not brake in a straight line. The garage checked my brakes on the rolling road, the same way as they check brakes when doing an MOT, it passed that okay so brakes were not the problem. I assume your garage would be able to do this simple brake check as well, this will show if both sides are braking evenly. Apparently, the setting up of the equipment to do a disc tolerance run out check, takes a while to do, which I guess is something that a garage will not freely do. They checked mine in the end (because I complained) and the NSF was miles out, with a new hub fitted!
To be honest i woudn't mess about, get the NSF assembly changed and put a new disc on that side only and see if that sorts it.