spenol,
As Evo’s quote points out you don’t need to re-do the exams since the NPPL uses the JAR exams in the first place. The concession allowing you to do only 15 more hours assumes that you currently have an NPPL with SEP rating and that all your training was carried out by a JAR-qualified instructor at a CAA-approved facility.
If your NPPL SEP was based on cross-credited microlight hours or if you only have a microlight NPPL you’ll have to do at least 35 hours more training, unless by unusual good fortune your microlight training happened to have been done by a JAR-qualified instructor at a CAA-approved facility.
Since you’re going to have to pay for several hours flying around with an instructor why not use the time to do your night qualification while you’re at it? You can apply for that at the same time as your initial application for the PPL. Might as well get as much as you can out of the time.
Apart from the hours, you’ll need to do the qualifying cross country flight and the normal JAR PPL skills test which is very similar to what you’ll have done for the NPPL, but includes more radio navigation and instrument work; specifically the 180-turn in (probably simulated) IMC.
You will of course need to get at least a Class II medical as well.