ESA Reduction based on private pension income

In 2009 I started claiming ESA following what everyone except Atos/DWP appreciated was to be a long term injury. I made it very clear that I was only interested in contribution-based NOT income-based. Interrupted only by 3 erroneous WCAs which were all corrected on appeal, the payments ran relatively smoothly. In Sept 2013 DWP told me they had being overpaying from the outset as I had been receiving private pension income. I had never concealed this fact and included it on tax returns etc. I am disputing their ability to demand any repayment on the following grounds:
1) The legislation that allows them to reclaim monies overpaid was in force prior to the introduction of ESA & therefore does not mention it. They say however that ESA is “deemed” to fall within the scope of the Act. My challenge is ”deemed by” whom? I have said that if the people who crafted ESA wanted it to fall within the scope of the older legislation, they would have made sure by amending it. The fact that they did not can only mean they had no such intention.
2) A bit of a side issue, but the numbers they have used are incorrect and it has proved very difficult to get them to reveal their sources to validate their calculation – it is now down to a SAR.
3) The fact that a contribution-based benefit contains an income-based calculation is hardly what one would expect. Over the past 4 years I have exchanged very many letters with DWP and never once has this adjustment been mention nor was it mentioned in any of the standard literature they published. They are saying that none of this matters as ignorance of the law is no excuse. My counter is that equally Government departments have a duty to make information reasonably accessible, which does not mean buried in their website or in the statute books. I therefore regard this as their error, not mine. Taking the pensions early was a borderline decision as it was and had I known that ESA would be reduced, I categorically would have left the pensions intact for my normal retirement date.
I feel I have a perfectly reasonable case to take to tribunal, but I would much appreciate some advice/comment from anyone more expert than me.

Thanks for the interest. I was able to start work in Sept 2013, so stopped my claim. In 2009 there was no EWA1, which looks to have been introduced about the same time in 2013, which would explain why, after 4 years of wrestling with DWP and been classified as “unreasonably persistent”, I had never come across it and they never mentioned it. The only form I was asked to complete was the ESA50 (3 times over). Over those 4 years I spent a huge amount of time on ‘research’, responding to Prof Harrington’s requests and corresponding with all and sundry within DWP and Westminster, I cannot believe that a similar form existed and I never came across it.
My simple analogy would be driving along a dual carriageway at a steady 50 mph, well below the normal legal limit (70 mph). The signs all say it’s a dual carriageway so you have no grounds to question the information. In fact you stop a passing policeman to confirm it is a dual carriageway and he tells you it is, not mentioning anything else. You are driving attentively and there is nothing to suggest you are breaking any law. Then the police stop you for speeding because in fact the limit is 40 mph. You protest as there are no signs to indicate that this is the case, not even a suggestion that you ought to churn through all of the local by-laws on the basis that ignorance is no excuse. Would you regards receiving a fixed penalty justified?