When the NPPL was first introduced the CAA managed to mess up the wording such that the SSEA rating did not have a fixed expiry date although it still needed the revalidation requirements to be met "before each flight".
This was changed a couple of years ago to bring it into line with other ratings hence it now requires a revalidation signature from an Examiner every 24 months. There was a period of grace available to holders of the "non-expiring"
rating in which they had to change over to the new system.
So, if you have an expiry date on your current SSEA rating, you will need to meet the revalidation requirements and have the rating page in your licence signed of by an examiner prior to the expiry date. Failure to do so will result in the need to renew the rating by a proficiency test with an examiner.
There is no requirement to fill in and return any form to the CAA as is the case with the JAR licence and sep rating.