My exhaust box outer skin is badly corroded on one side, but the exhaust isn't blowing.

The garage I bought the car from is saying this is normal for Cooper S's and the exhaust could still be good for years. According to the garage Cooper exhaust's don't even have a double skin, only Cooper S's, so the outer skin isn't necessary.

Am I being taken for a ride and is this going to be issue when it comes to the MOT?

Thanks, Richard