claiming money as had less than 16,000 for 1-2yrs

1/2Can someone advise to me as i have been asked by the DWP to write in to explain why i want to claim back payment as i had less than 16,000 for about 2yrs,and less than 6,000 since christmas, Having informed them of change in circumstance for ESA,after 4wks runaround ,then threatened action,within 1 day they gave me the full amount per week.
The previous months have been trying as i suffer from depression because of severe spinal problems which my specialist said i could be paralysed if i have a fall,this has thrown me and have not properly assesed my finances. So the gap between my money going under the two limits and me claiming is more than it would be if i realised the above.
On a legal stand point i think i should be entitled to the back payments as moneys got below limits in the last year or so,maybe the DWP will find excuses and blame me for not claiming sooner. Due to my illnesses my finances do get left,now with new medication my depression is improving,the pain never goes away but i am focusing more. Any advice on the above will be appreciated.

Thank you for your reply, i started claiming incapacity from 2002-2012,after an assesment it was moved to esa in a work support goup. at this time after looking after both parents sadly they past away and i had to sell the house to pay off their debts.
The money was split between me and my sister and brother,we all got 25,000,i told the dwp and they said i was not entitled as i had over 16,000. i did not recieve any money until recently after phoning in to reclaim, due to severe depression caused by decades of stenosis and 2 discs pressing on the central nerve in my cervical vertebrae i let my finances go and until recently when put on new medication i finally got myself together .
Having not realised that it was still an ongoing claim i phoned up on 2/06/2015 to make a new claim, they didn"t say the old one was still open so about the 21st of june they told me,i should have been told straight away,this has wasted 2wks, after making complaints they asked for bank details,i sent them by e-mail, they looked and on the 29th awarded me 102.00 ew, then on sat another letter stating that the 306,00 arrears was on hold as they were checking for overpayment, it also said they cannot pay esa from 2012 as i had 16,000+, the bank details sent clearly states i had less than 16,000 at march,2014.
At every turn they have made errors,not telling me of infomation to be sent,they are stalling on purpose to make me suffer,if i made any mistake or not they refuse my legal entitlement. sorry about the war and peace bit but i am angry and will seek legal action on individuals who lied to me.

Every phone call they seem nice, i ring again(they ring back on a 0800 line,i ring on a 0345 line) they always say i need to send more info,they are stalling paying my legal entitlement, it is done on purpose ,as all info is computerised and is clearly available.
Trying to get a first choice homes flat but there is at least 3 mnth waiting time, the rent i am paying is too much in this private rental, because of the delaying tactics of the dwp with very little money my brother said he would be guarntuer to another flat, at 100,00 cheaper rent i said yes, as i get council tax and housing only, because the dwp messed up and left me with no choice i will owe my bro 300.00 in fees. the day before i signed the esa came through along with delayed discresionary payment, which meant that if the dwp informed me straight away the issues would have been dealt with wks ago, and i could have afforded to stay until i got a fch flat,with no fees to pay.
Left out of pocket an let down by the dwp, i have no confidence in the bas---ds, i will question everything and seek compensation and apology or will seek the legal route.

Thank you for the advice,i found out that the person (decider) who looked at my details could not look at any letter sent by the computer,whether it"s true or she"s lying i don"t know,but if true on purpose the dwp restrict information at the front lines(phones) to on purpose drag out the process and cause hardship and worry.
This is illegal,setting up a system ,unless you chase them up ,delays are happening to the most vunerable,with no process flagging those with mental problems . It goes against our fundamental human rights under law, it is a pattern repeated ,understood to cause hardship, and like me what took over 3wks of nothing,only lies and excuses,after threatening with legal action in 1 day it all went through. without me pushing it would still be in a pile.
There must be thousands in my position with the dwp on purpose delaying payments,even after admitting(mistakes) and doing the same thing repeatedly.