Westy
I'm afraid the DWP's answer to your letter was misleading, ESA awards are indefinite (on-going) but are also subject to regular review and this is why you have received a new ESA50.
This is a full reassessment of your claim and you need to treat it as if you were applying for the first time, complete the form in detail and submit any evidence that you have, even if you have submitted it before. Whilst the ESA criteria has not changed, the DWP guidance has been updated so make sure that you use our latest ESA Claim guides to help you.
www.benefitsandwork.co.uk/help-for-claimants/esa1 If you have further questions then please reply to this post and we will do our best to help.
Gordon