I think there's a bit of scam going in some dealers. At the slightest sign of pitting or grooving, they tell you you need new discs. Had this on a Citroen, dealer said I needed new discs, I said no, I'll do it myself, but didn't; two years and two MOTs later no problems on the original discs.
My rear discs are now showing a band of corrosion - it passed its MOT in Feb like that, as the brake performance was OK. The trouble is the pads stick slightly in their guides and light braking is not enough to shift 'em. Every year I take the rear pads out and clean them up. Even so they seize in again.
The mpg goes up after a clean as well, as they then release properly. All rear-disced cars seem to suffer from this.
