The Definition of a Microlight aeroplane can be found in the Air Navigation Order Article 155
‘Microlight aeroplane’ means an aeroplane designed to carry not more than two
persons which has:
(a) a maximum total weight authorised not exceeding:
(i) 300 kg for a single seat landplane, (or 390 kg for a single seat landplane in
respect of which a permit to fly or certificate of airworthiness issued by the
CAA was in force prior to 1st January 2003);
(ii) 450 kg for a two seat landplane;
(iii) 330 kg for a single seat amphibian or floatplane; or
(iv) 495 kg for a two seat amphibian or floatplane; and
(b) a stalling speed at the maximum total weight authorised not exceeding 35 knots
calibrated airspeed;
Microlight Aircraft are certified in the UK to BCAR Section S, talk to the BMAA engineers if you need any specific information. The C42 can also be certified under JAR-VLA if the weight exceeds 450 Kgs, in which case it will be operated as a SEP aeroplane.