esa - not entitled to cb - decision letter

Hi sorry to bother you with this but i feel i need to do something about a decision letter i have just received to protect my PIP. I applied for ESA CB - i received a letter telling me i am not entitled to it. Fair enough I thought so i put the letter in the bin and have been getting on with life (not entitled to ESA IC as married), as I believed that was an end to it all though it stated on my letter that after assessment i could still get NI contribitions. I won't survive till pension age, i am 53 now and my health will assure an early demise. So when the assessment form came i put it straight in the bin. I have already paid 30 plus years NI anyways. No intention of putting myself through the stress of an assessment when i have to go through my PIP review later this year anyways. I believed the letter meant - i am not entitled to esa end of story i did not realise that my claim was active. Any ways today i have received a decision letter and the dwp have now closed my claim down but the reason that they have used is 'i am capable for work' which is rubbish i am not or why would i be getting full care and mobility. I do not understand how they can use this excuse to close my claim down as 1. my pip award 2. when i was in receipt of esa ic i was in support group and 3. they have never assessed me to reach such a decision. I telephoned DWP and asked them to change it to the correct reason of not being entitled to esa cb and omg what alot of idiots they are even after talking to a manager they insist that i am now capable for work. HELP how on earth can i challenge this - there has to be a way without wasting tax payers money and going through an assessment to prove i am not fit for work. I have tried to read the legislation but its a mine field i dont understand it. Even phoned CAB and they dont seem able to help and MP Gauke's secretary lol dont even ask they refuse point blank to speak about dwp and gave me a phone number to contact which when i phoned put me through to guess who - dwp esa. Pulling my hair out i just want the decision to read as it lawfully should in my opinion. Any help gratefully appreciated and thank you.

I don't want the ni reason already given. Will this decision affect my pip as I know both benefit linked. Still believe it unlawful to deem someone capable for work without assessing one as such. Can you direct me to legislation that states about dwp deeming someone capable for work please I've tried searching but it doesn't bring anything up. Thank you