If it's an NPPL, it will indeed be an SLMG rating, not a TMG.
The rating revalidation criteria are summarised in the NPPL FAQs page under "
How many hours each year do I need in order to keep my NPPL current?".
Without knowing how many hours you've done since gaining the licence I can't say how much you still need to do to revalidate. When you have additional ratings, you need to do a minimum of 1 hour P1 in each to keep it valid. All the other requirements can be a combined total across all classes, ie hours in an SLMG will keep the SSEA valid as long as you do at least 1 hour P1 in an SSEA.