historic correction to SRP - recoverable o/p or not? 

Hello

We got an ATLAS for a female pensioner, updating her SRP right back to Decemnber 2009….. this calculated automatically and created a £3.5k o/p
Customer miffed! says Shes heard nothing from DWP and not had any additional/lump sum payments from DWP

My first thought was that it was one of these cases that has been in the news, where SRP based on married womens NI stamp and husbands NI was calculated wrongly for years and they are now putting them right.

If it was such a case,
1) how the dickens would we know? ( there’s nothing on cis notes) and
2 would it be a recoverable overpayment, or DWP error o/p or
3) would the arrears be treated as capital because of DWP error (can’t remember the figure of the top of my head or what benefits can count as disregarded capital but hoping you know what I’m talking about,)

Anyone know of any super duper short cuts/neat tricks that might help?
Thanks - Prisca