it has to be to the BMAA syllabus, but can be a JAR-FCL FI(A)
Actually I went through the same process myself a year or so ago and queried this with the BMAA. They referred me to their licensing expert who was adamant that it had to be a microlight instructor. I was only interested in 3 axis so the "differences training" was in reality a joke 'cos there weren't any.