Backdating

Hi,
I wondered if someone may be able to point me in the right direction -
Long story short, cl claimed WTC in Nov 2016, later awarded PIP (24/04/2017) which was subsequently backdated 7 months, due to condition did not notify HMRC until 03/06/2017 and therefore disabled worker element backdated only 31 days not the 7 months to the date that PIP was actually awarded from.
Obviously as cl did not notify within the time limit they weren’t automatically entitled to backdate, however HMRC did send cl a letter at the beginning of May 2017 stating that they had been notified that they were receiving PIP and this may now entitle them to receive the disabled worker element and they must reply by 04/06/2017 to get this element, however they knew that the key date for full backdating was before this date and had this have been highlighted on the letter client states they would have responded quicker.
Do they have any grounds to request they backdate further or should we just plead to their better nature?
Mandatory recon has failed, so now looking at appeal.
Thanks in advance

you can get a backdate under official error in tax credits but its 2 step.

you have to show the error and then that your client didn’t contribute to it (which basically covers hmrc for lots of sins as its in the guidance usually….that no one ever reads through).

id say in this case though it would be harder to argue client contributed?

Yes, I’m concerned that this isn’t official error criteria.
Does anyone have any experiences arguing this type of case?

Just wanted to post an update - appealed the decision but no long after this was registered HMRC then changed their decision and awarded the cl the full backdating. So a good result!