ESA Overpayment - Mandatory Reconsideration Notice

Hi All
I have an urgent dilemma regarding an over-payment of ESA. I was placed in the support group for ESA having applied for it on the advice of an ESA Advisor at my local Job Centre back in 2011. I am 60 years of age and have worked full time since the age of 18 years, apart from one period of maternity leave lasting 4 months. Unfortunately, in early 2010 I became unwell following a surgical procedure. I was in and out of hospital over the next year and attended frequent out patient appointments. In mid 2011 my employer, an NHS Trust, decided that I was unable to continue working in the capacity that I had been employed and I was forced into applying for early retirement from the NHS on the grounds of ill Health which I was awarded in late 2011. I did make the local Job Centre aware of my claim for early retirement even before I was made aware of the outcome but once the paperwork arrived I did contact the Job Centre to arrange an appointment to provide my updated information. It was in the run up to Christmas and so there was a longer than usual wait for an appointment and I finally attended on 29.12.2011. I provided all my information about the NHS pension award which included details of a lump sum, the monthly payment at that time (including back pay paid due to delays in payment beyond my control) and also the fact that an annual increment would be applied to the monthly payments. At this visit I was advised that I should appeal a decision to put me in the Work Related Activity Group for ESA and request reconsideration of my circumstances with a view to being placed in the Support Group. After much confusion along the way in terms of errors being made by DWP e.g. incorrect forms being sent out to me which meant that I had to start my appeal again and then delays due to my mail being sent to the wrong address etc I was finally placed into the support group. I provided the information about my NHS Pension award again during this process on at least 2 further occasions.

In October 2017, I received a letter from DWP stating that I have been overpaid due to a failure to disclose the fact that I was in receipt of an NHS Pension. I was naturally mortified. After much upset and resulting stress I was finally able to prove that I had indeed provided the information on several occasions. I submitted a request for a Mandatory Reconsideration and on 30.01.2018 I received a telephone call from a Dispute Resolution Team Representative who told me that my request was unsuccessful on the grounds that I had not informed the office that paid my benefit straight away that I received my NHS Pension - not strictly true as I did not get the money straight away. Also, apparently I should have told the office that paid my benefit every year that I had received an increment. I genuinely did not realise this because this information was included my original paperwork submission. I understood a 'change in circumstances' to mean an income from an alternative source. When I said this to the lady who called me she said "I know our information letters and booklets are very confusing" I said I thought that the majority of the public would interpret things the same way that I had and she agreed with me.I was really upset during the phone call and the lady said " I am so sorry I understand how you feel" I said " I'm sure you mean well but I doubt that very much - DWP staff have said it to me quite a lot lately" her reply was astounding - she said " No I actually do - I have an autistic son and I am currently in the process of repaying an over payment, Even I got it wrong and I work for the department".
So - my dilemma is Do I go to an Appeal Tribunal to dispute the recover-ability of the over payment, which is in excess of £9000 due to the length of time involved. I am disabled and suffer from a number of painful chronic conditions as well as problems with anxiety. I really feel strongly about this as I have not done anything deliberately wrong in terms of withholding information. This is a lot of money and at the moment I am not even sure how the figure has been arrived at. I have today requested a breakdown of how this figure has been arrived at and am trying to get an appointment at the Citizens Advise Bureau.
I would be very grateful for any advice that anyone could offer.
Thanks in anticipation

Thank you for your reply Gordon. I did tell DWP at the time and following the Mandatory Reconsideration they acknowledge this. However, now they are saying that I should have taken the documents down to the Job Centre before my meeting with the ESA advisor (I actually made the appointment to do that and to see what else I needed to do) also that I should have given them an annual update on my pension amount. I genuinely thought that the fact that my paperwork said that there would be an annual increment and also that I know that DWP and Inland Revenue are in constant contact would be sufficient for them. My 'error' is not an intentional act of fraud - just a misunderstanding of the requirement year on year.
thank you