Unfortunately, some total idiot at the CAA decided not to issue NPPL Ratings with 'valid until' dates. The ANO states:
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.
So your rating will remain valid so long as you continue to observe para (i) above. But you turn into a pumpkin as soon as the last 1 hr flight with a FI was more than 2 years before the date you intend to fly.
Utter nonsense, isn't it. Which is why we've been trying to get it changed for nearly 2 years now since the original CAA cock-up.