The CAA made a complete balls-up when they introduced the NPPL; someone wrongly used the 'ballonists' validity requirements totally contrary to the NPPL Policy and Steering Group's decision. They also cocked up the Rating privileges for the SSEA Rating, saying that you could fly microlights and SLMGs on a NPPL SSEA Rating having only completed 'differences training'. Again, this ran contrary to the NPPL P&SC's decision.
So we've been trying to sort it out for nearly 2 years now - because at the moment:
Section 3 – Maintenance of validity of National Private Pilot’s Licence (Aeroplanes)
1 A simple single engine aeroplane (NPPL) class rating included in a National Private Pilot’s Licence (Aeroplanes) shall not be valid for the purposes of article 24A unless the provisions of this Section have been complied with.
2 (a) A simple single engine aeroplane (NPPL) class rating shall be valid if either:
(i) the holder has within the 12 months preceding the flight flown not less than six hours in an aeroplane falling within the simple single engine aeroplane (NPPL) class rating, four hours of which shall have been as pilot in command and he has carried out a training flight of at least 1 hour duration with a flying instructor within the previous 24 months; or
(ii) he has within the three months preceding the expiry of the rating undertaken a simple single engine aeroplane (NPPL) General Skills Test.
But there's no such thing as a 'rating expiry date'! Technically, it is impossible to 'revalidate a SSEA Rating if it doesn't have an expiry date!
It will change so that the validity is going to be for a 2 year period with a defined expiry date - you just need to meet the revalidation requirements before that date and the Rating will be revalidated for another 2 years from the expiry date.