Firstly, somebody has already come in and criticised use of the term "Group A", so I'll simply say I couldn't care less and we all know what you mean.
Assuming that you have SEP currency, Microlights for your JAR CPL, and I think your UK PPL now (although not previously, the rule became retrospective) requires differences training only.
In essence you need to fly a suitable 3-axis microlight with a suitable instructor (microlight instructor, or a JAR instructor with microlight currency) until they are happy and prepared to sign your logbook for the differences training.
This differences training can be the type checkout with an instructor anyhow and that can be your differences training flight at the same time with no extra cost. There is no minimum length of this training - half an hour may well do it.
That's it - have fun. List of microlight clubs and schools in your neck of the woods
here; I doubt you'll find a bad microlight school, they're all very much enthusiasm led. Do be aware that if you plan to fly some of the older very high drag/low interia, 2-stroke engined microlights a lot of the newer instructors have never flown these. The newer 4-stroke engined aeroplanes will be much closer to your lighter "group a" experience anyway and the checkout would be a pure formality.
As Whopity quotes, microlight flying for you will require SEP currency but the microlight hours won't count towards your SEP recency requirements.
G