BMAA Pilot Training might be very useful - it shows the given standards, syllabus, checks, etc. in microlight training.
I have about 600 hrs on mirolights, split roughly equally between 3-axis and flexwing if you want to pick my brains on the flying of them, rather than necessarily the instructing on them (still sitting on 0.000 on my CRI rating, unless you want somebody to do your microlight differences training for you, in which case doing that for an experienced instructor would probably be great for both of us).
You do need to standardise on microlight checks and know the syllabus, after that a "hot ship" microlight such as a C42 or Eurostar will not challenge an existing FI; the older more "pure microlight" aeroplanes like the Thrusters or an AX3 will challenge you as quite different to anything you're likely to have flown before and I'd argue that you want to spend significant quality time in them before attempting to teach.
Microlight air law contains stuff (pilot maintenance, PtF regulations...) not met in normal JAR air law, and to a lesser extent so do the technical and met subjects. You really want to know this material.
For most of the above, I'd recommend the 2nd and last books on
this page which should help a lot.
As a fairly experienced 3-axis pilot, when I switched to flexwings it was probably 30-50 hours before I stopped scaring myself on every flight and feeling on top of the aeroplane, which is perhaps a reasonable starting point for thinking you *might* be able to instruct on one. If sticking to 3-axis, and the more modern 912 engined 3-axis microlights you can reasonably do so fairly quickly - although 15 hours seems a sensible minimum to me, regardless of legalities.
G