Hi moneymattersteam
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 .
Usually you cannot backdate a PIP claim form but you may be able to ask for an anytime review due to an official error, the information you were given was incorrect.
We would advise you to seek help from your local Welfare Rights Specialist; you can find your local Welfare Rights Organisation to help you with advice by clicking on the link below and putting in your post code, it will then come up with a MAP with a list of agencies in your area
advicelocal.uk or
www.lawcentres.org.uk Gary