Andy H is I think correct.
There used to be microlight models and group A models. When the "new" 450kg microlight definition came in most, probably all, Minimaxes became microlights. This seriously disadvantaged some group A Minimax owners who had been maintaining their licences quite legitimately on these aircraft.
So, PFA and BMAA did a deal with the CAA on behalf of their members that group A Minimax owners could continue to keep their licences current on those aircraft for as long as they owned them. However, I don't think it was ever going to be possible for any new pilot/owner to take on this privilege as they started flying the aeroplanes after the rule change.
Still a lovely little aeroplane; I'd certainly seriously consider a share in one, if there was a local opportunity (and I could persuade Mrs.G that I really did need a fourth aircraft share to add to the three I've got already!).
G