Either:
1. renew the SEP rating now, by passing the test sending in the relevant paperwork and relevant fee you will get a JAR FCL licence and then be able to do microlight differences training on that rating
or
2. If you only ever want to fly microlights, forget your current licence and do the training for an NPPL(M)
Are you planning to fly 3-axis or weightshift microlights? If weightshift you will probably need at least 10 hours differences training anyway, probably even more.