Help with backdated payments after winning appeal

Hi first time I've been here so please excuse if I babble on but I try to just use facts (as I understand them)
My son put in a claim for ESA in May 2011 DWP today confirmed it was 23rd May , he had the ATOS medical in October 2012 & failed it with no points. He suffers from depression & some days are better than others.
He appealed & Tribunal was Feb 2013 they awarded him 18 points, so he won his appeal.
We heard nothing from DWP so I rang them 2 weeks ago & they said they had no record of his appeal which was a lie as they had sent the paperwork back to Tribunal because it had my son's NI no wrong, so someone was clearly doing something with it.
Last week he had a letter stating he would receive backdated payments to March 2012, that didn't seem right because he first claimed in May of the previous year.
I rang DWP today to query this & was told he was only entitled to backdated payments from 31 March 2012.
I'm sure I've seen here & elsewhere that any difference between what he was paid & what he should have been paid less 14 weeks was his entitlement.
Can anyone help, am I right, is DWP right ? if not what do we do now ?
Thanks in advance

Hi Carole and Welcome to Benefits and Work

Gosh, I have never heard of such a long wait between application and having a face-to-face assessment! Was your son being paid assessment rate ESA during this time (I am assuming that this was a new claim for ESA, not a migration from IB)? You are right in saying that backpay should consist of the difference between the assessment rate and the rate for either WRAG or Support Group (I suspect the WRAG from what you have said) from the beginning of week 14 of the claim.

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