Well first of all you are going to have to prove that the rust was there at the time of the MoT and that is was in an advanced state and should have been a failure. You will only be able to do this with the help of a laboratory that specialises in these things, the opinion of another garage is of little use I'm afraid.
Any problems in this area which may not have been a problem at the time or warrant a refusal to issue a certificate would have been noted on the 'advice' sheet, do you have that and what does it say.
Don't repeat 'hearsay' from friends or friends of friends about the integrity of the MoT station or you might land yourself in trouble.
Trading standards can (but not necessarily will) help in this. They can take in a doctored car and see what the outcome is, so give them a ring and see what they have to say.
If you don't have a copy of the advice that goes with the MoT (some garages don't give it if it could advise of future problems), then go here and enter the details to see what it says:
MOT Information - Introduction