Back to the original question.
I have a EASA PPL & am an unrestricted instructor. I have had Microlights differences training on 3 axis machines. However when I asked for my NPPL to be amended to include a microlight class rating the BMAA confirmed that I would have to pass a GST & oral exam before it could be issued.
So I can instruct on microlights with my EASA licence but am not entitled to a class rating on my NPPL without a test!