This "is" a can of worms. Seems the more you complain the more gets sorted.
I don't see why ESA should come into it if you get Unemployability Supplement.
It only comes under ESA rules now as they linked it to Incap. I think it can be payed as Serps or Additional Pension as it is.
I was told I would be payed SERPS whether I took state pension at 65 or not. I was told it would appear as SERPS!
If they can pay it then, they can pay it as that now.