Nothing - just 'differences training'!
LASORS seems to have a different opinion (section F1.1):
Please note that the JAR-FCL SEP (Land) Class Rating (which replaced the former “Group A” rating) does not include the privilege to fly microlight aeroplanes. A SEP rating holder requires a separate microlight rating to fly aircraft in this category. For further information, individuals should contact BMAA, please refer to Section A Appendix E for contact details.
The NPPL web site suggests that at most you would need to do the microlight oral exam and flight test but I haven't found anything that specifically says how you go about getting a microlight rating on your JAR-PPL.
Hmm...
Mark