Beamer, PPL to fly microlights:
If licence is the OLD style CAA licence from pre JAR days, then you're already legal to fly microlights, of course a check out being recommended.
If you have a modern licence, EASA Part FCL with SEP privileges, then ANO Article 150(6) states: "A Part-FCL licence with single-engine piston aeroplane privileges is not deemed to be rendered valid for a microlight aeroplane unless the holder of the licence has undergone differences training in accordance with Chapter 2 of Part 2 of Schedule 8, appropriate for a microlight aeroplane class rating."
So get checked out by an instructor and when he is satisfied have your logbook signed accordingly by that instructor and you're all set.