The Mirror are really slow, as this has been rumbling since 2011. The Naval personnel affected have mostly been warfare and, in most cases, were actually aware that they had been overpaid. Also, for the Mirror to say that the rules for AIP are complicated: It's not. The JSP 754 listed the eligible qualifications by branch, so in the event of any questions, it was there for all to see.
It should also be pointed out that RAF personnel ran to the press over this issue in the first instance, squealing like a greased up pig, rather than go through official channels. The RN personnel affected went through official channels, submitting PACCC casework, detailing why they shouldn't be paying it back (the competences were very similarly named in their case) and a fair few didn't have to repay it. Those that do, possibly didn't submit a strong and compelling enough case.
Oh, we did also encourage them to submit service complaints because of the uncertainty of whether they would be paying it back or not, while SPVA and MOD blamed each other as to who had to take the decision to write. I believe it was SPVA as it was ultimately their failure (and hence the later change of name to DBS).