Golf Brake Discs - Bhuijn
My 4 year old Golf GTI has just failed its MOT due to 'excessive pitting' to all of the brake discs. The brake efficiency and balance passed OK.

When I was shown the discs each one had 10-15mm of corrsion on the inside of the discs near the edge.

Was the failure warranted? I have I got grounds for appeal?

They (a VW main dealer) also wanted nearly £400 to replace the discs and pads so I walked away anyway.
Golf Brake Discs - Roger Jones
Don't know about appealing, but you can walk away from a VW dealer and find one of the many independent VW specialists to do the job for you. Take it along to one and see if they think it's worth appealing. If they are honest and say yes, you'll go back to them for further work. If they're honest and say no, they'll explain why in a convincing fashion (apart from their wish to acquire work, for which you can make due allowance).

Has the car been left standing a lot? At four years of age the need to replace disks is perhaps a bit surprising, but it does depend so much on how it's been used.
Golf Brake Discs - Bhuijn
I have done 32.5k miles (about 10k each in the previous 2 years). It does get left on the drive occasionally but never more than 2 weeks at a time.

I did get the the opinion of another guy who runs his own small garage. He said he wouldn't have expected them fail, he would have just advised about them. He is also a lot chaeper as you would expect!
Golf Brake Discs - Glaikit Wee Scunner {P}
My Golf MK3 and MK4 both failed their first MOTs on similar grounds at the main supplying dealer.45-50k. I complained both times about poor VW design/quality and got a substantial discounts on replacements.
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I wasna fu but just had plenty.
Golf Brake Discs - Bhuijn
I had the car retested today by testers from VOSA. It passed!

So I guess you can't always take the tester's word for it.