I am clearly no legal eagle, but there is a law concerning misrepresentation: If I buy a car based on the fact the salesman promised it would have a 6 year warranty and then found subsequently that it only had three, I could claim to be compensated for misrepresentation, ie I bought the car based on a particular promise.
In the case of AIP, Service members applied for an AIP based on a promise (policy) by the MOD that they could jump a pay level if they possessed certain qualifications. Now, ten years later, those that took up the AIP offer find out that they should not have jumped a pay level after all and must pay back the difference.
Surely they have been misrepresented by the MOD, ie they entered into the AIP programme based on a promise, and therefore have a counter claim against the MOD (presumably for the sum they are being told to pay back)?
Just a thought.