It really isn't that difficult...
If you wish to fly a SLMG, in your case you have 3 options:
1. Maintain a valid SEP Class Rating and exercise SLMG privileges by taking relevant differences training.
2. Add a TMG Class Rating by passing a TMG Class Rating Skill Test.
3. Forget all about JAR-FCL and obtain a NPPL with SLMG Class Rating. See the NPPL website to find out how to convert your licence.