Your PPL(A) and a JAR SEP give you the right to fly a microlight - any type - without further training, tests etc.
Reason is that the ICAO definition of a SEP includes microlights - or rather, does not exclude them!
Whoops. Just a tad too fast.
ICAO and JAR-FCL are nice documents, but in the UK it's only the ANO that applies. And the ANO makes a specific exception for UK-issued JAR-FCL PPL(A) wrt. microlights. I could not find the reference in the ANO quickly, but here's what LASORS has to say about it:
The holder of a UK JAR-FCL licence with SEP rating
may also subject to completion of differences training
with an appropriately qualified flying instructor, exercise
the privileges of their licence on microlight aeroplanes
and SLMG’s in UK airspace only, without the necessity
of obtaining a NPPL (the normal licence for such
aeroplanes). However, any experience gained in
microlight aeroplanes or SLMG’s cannot be counted
towards the flying experience necessary to revalidate
the SEP rating.
[LASORS C1.1]
So by law you have to obtain appropriate differences training by a qualified instructor, who then has to sign you off properly.