Rust on brake discs

[JAZZMAN2006]

I have a 2006 Jazz automatic. Last year it passed its MOT with an Advisory that the brake discs were corroded but not seriously weakened.

 

This year at the same garage (but in the absence of the owner who was not well) it failed its MOT and the discs were classified as Dangerous. However the car had only done 200 miles from the previous MOT. Is this plausible given the low intervening mileage and the report in the previous year's MOT ('not seriously weakened')?  

 

In any case, couldn't the rust just be cleaned off - or are brake discs so cheap that one might just as well replace with new.

[sceh]

Dangerous? BS

All discs rust and the ONLY criteria of danger or not is thickness and it needs to be pretty thin to be dangerous. If they have only worn down, say, 1mm, they are not dangerous and rust is not dangerous. Brake hard a few dozen times and they are clean. I am assuming they are talking about the surface of the disc