Thanks to Nick Dilworth who wrote this template letter
You can follow him @Mylegalforum http://ilegal.org.uk/
The Customer Complaints Manager
[Address of your Employment & Support Allowance Office]
*shown on your most recent correspondence from them
Dear Sir / Madam,
National Insurance No:
Arrears of Employment & Support Allowance which may be owed to me through your official error
My attention has been drawn to some recent media coverage by the BBC entitled ‘Mistakes in benefits claims could cost up to £500m’ (dated the 17th November 2017). As a result of this I am making an official complaint, I believe I may be affected and entitled to arrears of Employment & Support Allowance.The media coverage states:
“The errors identified by the Department for Work and Pensions affect the main sickness benefit, the Employment and Support Allowance (ESA). The BBC understands that assessors wrongly calculated the income of around 75,000 claimants. Ministers say that they are aware of the problem and that repayments have begun to be made. The department, which says it discovered the mistakes last December, is understood to have contacted about 1,000 people so far. It says it is still trying to understand the scale of the problems with ESA, which is paid to about 2.5 million people, and will contact anyone affected.”
Having made some enquiries, I understand that I may be an affected individual because I underwent a ‘conversion’ assessment from older Incapacity Benefits / Income Support paid on the grounds of incapacity for work / Severe Disablement Allowance. My recollection is that this was subject to a decision made by you on or about the [insert date].
The conversion process should have been carried out in accordance with the Employment and Support Allowance (Transitional Provisions, Housing Benefit and Council Tax Benefit) (Existing Awards) (No.2) Regulations 2010 and it is my understanding that in confirming your decision you should:
(a) have ensured I was entitled to the correct amount of ‘transitionally protected’ benefit at a rate whereby my converted Employment & Support Allowance award was equal to what I received before conversion and should have continued to be protected on a ‘mark time’ basis until the level of Employment & Support Allowance either caught up with my transitionally protected amount or until the 5th April 2020.
(b) As part of the conversion decision making and notification process you should also have checked any existing contributory award to see if I may qualify for an income based amount on the converted Employment & Support Allowance award as confirmed by the Upper Tribunal in  UKUT 342 (AAC) where it was held [In considering Regulation 8 of Employment and Support Allowance (Transitional Provisions, Housing Benefit and Council Tax Benefit) (Existing Awards) (No.2) Regulations 2010]:
“Given the unified nature of ESA as described in paragraph 25 above, the determination by the Secretary of State of the amount of ESA to which a person would be entitled on conversion under regulation 8(1) in my judgment must encompass consideration of both the contributory amount and the income-related amount.”
I am therefore asking you to confirm that neither of the above omissions (a) or (b) occurred as a result of any official error on my claim. If official error has occurred and I have been underpaid in consequence of that error, please ensure that you pay me any amounts owing to as a matter of urgency and without unacceptable delay.
Please note that this request is an official one which you must treat by way of an anytime review or supersession request.
I must further point out that this request applies to a retrospective decision and therefore applies in cases where there may have been a subsequent successful appeal against any initial decision to refuse or otherwise restrict the Employment & Support Allowance award made. Likewise, the fact that I have since stopped claiming Employment & Support Allowance or taken up another claim to other benefits does not prevent me from making this request.
So that I can check the accuracy of your records please treat this letter as a Subject Access Request and supply me with copies of my pre – conversion awards and all claim details pertaining to my Employment & Support Allowance claim from the point of conversion of my claim.
Please also consider this as a complaint of potential maladministration on my claim and consider issuing me with an appropriate compensatory or special payment.
On a final point, please make me aware of the effect which this may have upon any other benefits such as Housing and or Council Tax Benefit paid at the point of conversion.
I look forward to hearing from you and trust that you will look in to and act on this request as a matter of urgency.