No - you don't have any different rating validity requirements for flying a microlight on a JAR-FCL PPL(A) licence with SEP Class Rating. You should complete differences training to qualify, then as long as your SEP Class Rating is valid you may fly microlights using those privileges.
If your SEP Class Rating lapses, you cannot fly microlights on a JAR-FCL PPL(A) - you would need to obtain a NPPL with Microlight Class Rating if you didn't want to revalidate your SEP Class Rating.
Don't forget that you can always revalidate your SEP Class Rating in the second 12 month period by flying a LPC in the final 3 months - which could well work out a cheaper option if you can't afford 12 hours of SEP Class flying.
Finally there is no such thing as 'Group A'!. I wish that certain flying associations would grasp that concept - as it's mostly their out of date terminology which breeds confusion!