ESA

All about money
It might be worth noting here, that if you don't apply for anything from DWP because you didn't know about it, that's tough luck, usually they will not backdate it.
However, if DWP have mucked up, then under certain circumstances they will backdate it and even pay compensation.
So the first step is to find out what went wrong. When I ring DWP the first person I speak to is usually only familiar with the basics. At some point, they will say there's nothing more that they can do. Then I say "Please may I speak to your supervisor" and then ask the same question again. I may even ask to speak to the supervisor's supervisor.
It's really important to have a pen and diary handy, to make sure you know the name of anyone you speak to at the beginning of the conversation. Under their rules, they should never refuse to put you through to a supervisor.

(I know someone who used to work for DWP at a fairly high level, so whenever I've had a problem with them, I just ring my friend for advice!)
My council until recently had a Welfare Rights Unit.

Just before I reached age 65 a social worker told me that she thought I may be eligible to get PIP.
and she would make a referral to the Welfare Rights Unit for me.

After about 7 or 8 weeks I decided to visit the SS office which housed the Unit.

The receptionist behind her glass screen told me NO.

Anyway I found out the number of DWP PIP claims and phoned them myself directly, just about 14 days short of my 65 th birthday. It is fortunate that I did submit my claim myself just in time.
I was awarded PIP.

But I followed up with the Welfare Rights Unit and they gave me compensation for their delay and muddle over the referral.