Very complicated. I was awarded Incapacity Benefit in LATE

Very complicated. I was awarded Incapacity Benefit in LATE April 2001 and this was paid to me until I reached 65 on 28 May 2013. At no time was I advised PERSONALLY that any private pension over £85 per week would reduce my claim by 50% of the amount greater than £85. As a result I owed £10564.99 for the period 18.08.2002 to 16.10.2012 when the ERROR was eventually discovered. They are now deducting £12 per week from my state pension and there is NO tax relief on this £12. I have claimed and received the overpaid tax for years beginning April 2009 through to April 2013 but I am unable to claim the earlier years. As their claim did NOT include a penalty, am I justified in my view that one government department owes another the amount of overpaid tax for those earlier years and also that I should NOT be being taxed on my repayments. My weekly income from all sources is £8 greater than my expenditure, £255 and £247. I simply cannot afford any increase in payments that they regularly ask for. Every year HM Revenue has issued a tax code to my private pension provider with notice to me showing the amount of benefit I received from the Incapacity Benefit. So how can one department be aware of the private pension yet the other not and that continued for 10 years. I was divorced in April 2012 and this debt was NOT known at that time. As all Incapacity Benefit went in the 'family pot' could I have a future claim on my ex wife's estate for half the amount of the original debt. Don't know which way to turn and cannot afford expensive legal fees. I am single and have NO family whatsoever. Regards ***** *****