If you hold a UK/JAR-FCL PPL(A) with SEP Class Rating, to exercise Microlight aeroplane privileges you merely need to complete differences training and do not need a NPPL. Contact a Microlight club to establish what your 'differences training' would consist of.
However, hours flown on Microlights will not count towards the hours needed for revalidating your SEP Class Rating.
See LASORS F6.1 and
NPPL