I have looked up the rules and this is what I found courtesy of Cosgrove - The Microlight Pilots Handbook.
A) Always Clear of cloud and insight of the surface
B) Always have in flight vis greater than 3KM
C) Always when above 3000ft - viz not less than 5km - never less than 1500m from cloud horizontally - never less than 1000ft vertically from cloud.
All of this applies outside of controlled airpspace.
Usual permit rules also apply.