Hi Chris,
Basically you are looking at a two stage process:
NPPL(Microlight) -> NPPL(Simple Single Engined Aircraft) -> JAR-FCL PPL(A)
Although as stated earlier, there is not much benefit in going from NPPL(SSEA) to JAR-FCL PPL(A). LASORS is your friend for all this sort of stuff. You can get a free down-load .PDF (75Mb) of the document from the CAA website, which saves buying it.
LASOSR view / down-load
Section 6 refers to the NPPL, with Section C6.3 giving the details for the NPPL(M) licence.
Look at the NPPL website and check Section 2.1 - Cross Crediting for NPPL(M) to NPPL(SSEA)
NPPL Cross Crediting Document
Now jump back to LASORS Section C6.2 for a subsection on
Upgrading to JAR-FCL PPL(A)
LASORS isn't exactly good bed time reading, but combined with the NPPL website stuff, it will answer your questions.
Good luck & Safe Flying,
Richard W.