Well, you can't really expect the IAA to be completely up to speed on the rules that the CAA applies, can you?
Within ICAO and JAA, there is no definition of a microlight, or an exclusion of the microlight class from the SEP(A) class. In principle, your JAR-FCL PPL SEP-A thus allows you to fly anything with fixed wings (not rotary) and a single piston engine (with a few specific exceptions where type ratings are required, such as the PA-38). That includes what is commonly known in the UK as a microlight. But in reality the "microlight" class only exists within the legal framework of the NPPL.
It is advisable to obtain differences training though, and have this differences training signed off by an instructor. That's the advice the CAA gives. At least, this is what it says about microlights in LASORS 2007 section C1.1:
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.
Now the meaning of this paragraph is a bit unclear and it's been discussed before. This paragraph does NOT come directly from the ANO, suggesting that it is entirely legal (though stupid in case of a weightshift) to jump into a microlight without differences training. It also does not make clear, in this context, what an "appropriately qualified instructor" really is. And it is also not clear where the legal basis for not being able to count your hours towards revalidation is coming from.