From the web site
www.nppl.uk.com
SECTION 4. CROSS-CREDITING LICENCES AND RATINGS TO NPPL (MICROLIGHT)
4.1 Pilots with valid licences and ratings
NPPL (SEP), UK PPL (A) or JAR-FCL PPL (A) to NPPL (Microlight)
The holder of a NPPL (SEP), UK PPL (A) or JAR-FCL PPL(A) with SEP class rating who wishes to obtain a
NPPL (Microlight) shall:
a. Produce the NPPL (SEP), UK PPL (A) or JAR-FCL PPL (A);
b. Pass the Microlight Aeroplanes Type (Part 2) oral examination conducted by a Microlight flight
examiner. The examination shall include pilot maintenance requirements and conditions of the Permit
to Fly;
c. Hold a valid NPPL Medical Declaration or JAR-FCL Class 1 or 2 medical certificate;
d. Pass the NPPL (Microlight) GFT.
But that's only to get you a NPPL! Question is, does your FI rating count for anything after that? I.e. can you instruct on microlights?