Hi mr ch
Welcome to the forum.
You might want to have a look at the following FAQ which explains where everything is;
www.benefitsandwork.co.uk/guides-for-claimants/faq/forum.
I hate to bring you bad news but unless you reported a change of circumstances then you will not receive a backdate to December 22, it will be from when a decision was made.
An enhanced disability premium applies to IR ESA, you can receive the premium if you are placed in the support group or receive enhanced daily living component of PIP.
Backdate payment of benefits are usually ignored for one year.
When your savings go over £16k any means tested benefits you receive will stop, between £6k and £16k for every £250 bundle or part bundle you loose £1.00 in tariff income.
We would advise you to seek help from your local Welfare Rights Organisation;
advicelocal.uk Gary