My 2006 Skoda Octavia has done 21,000 miles and is still under warranty. One of the rear brake discs has developed rust over part of the surface, evenly distributed round the circumference and on the hub side of the face. The local agent assures me that the calliper is working normally, but also tells me that this is a typical way in which brake discs fail. The next service is due in 6 months and he suggests that it may well be appropriate to change both rear discs and pads at that time, for the princely sum of £195. Is there a method of recovering the disc economically? Should I replace both discs plus pads, or only the one affected?