As others have said take care here because out of the NPPL comes beaurocracy beyond belief.
There are two motor glider ratings. TMG which is a JAA rating and which can easily be added to a JAA licence. There are very few schools equipped to train for this rating which requires a minimum of 45 hours, a Public Transport C of A and a licensed aerodrome.
Then comes the SLMG rating, a National rating which is now part of the NPPL. This is conducted under the auspices of the BGA at unlicensed aerodromes. It is much cheaper, more readily available and requires only 32 hours.
Now to convert your NPPL (SLMG) to a NPPL (SEP) rating, you only need differences training. There is no test according to the NPPL Website. This means you can acquire a SEP rating to carry up to 3 passengers without having been near a JAA instructor or Examiner (other than for the undefined differences training).
To convert you NPPL to a JAA PPL will then require 25 hours with a JAA instructor.