Hi Oscarab
I don't think it has anything to do with your extension, it may be due to LEAP review. If you asked them to look at your claim again and found in your favour then your claim should be backdated.
I would contact your local Welfare Rights Organisation and show them your letter. 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.
advicelocal.uk Gary