Sub-standard brake disks on XJ?

Hi group,

My father-in-law has a 2004 XJ 3.0 V6 SE SWB. It's just failed its MOT,
the reason being pitting on the brake disks, and my F.I.L. reckons
that's a poor show since he's only done 2452 miles since the last MOT.

The garage reckons it's because he's not driven it much, but should it
really be the case that this would cause pitting on the brake disks? I
could understand surface rust forming if the car isn't driven for some
time, but pitting suggests a sub-standard metal used for the brake disks.

Any opinions? Any experiences?

Regards,

Patrick