Hi,
IAA is wrong.
UK PPL (A) and JAR (SEP) are ICAO compliant licences.
ICAO has no definition of a microlight, instead using a maximum weight of 5700kgs.
So microlight weight aircraft can be flown on ICAO compliant licences without differences training. (the international guys have not come up with an international definition to be able to exclude them!)
Some aircraft - like Tipsy Nippers - weight less than the UK microlight maximum of 450kgs but don't meet the UK wing-loading or stall speed requirements so are still light aircraft, for instance.
The UK NPPL is not an ICAO compliant licence, so the UK CAA can make their own rules.
Thus with an SSEA (the NPPL equivalent of an SEP, -ish!) you cannot fly a microlight. With a microlight NPPL you can only fly the type you have trained on unless you have differences training (ie weighshift or three-axis).
Hope that helps,
XA
ps. However, microlights don't count for revalidation hours for JAR or PPL (A) licences. Work that one out!