Appologies - I've used the wrong wording.
I should have said "Differences Training".
http://www.caa.co.uk/docs/33/CAP%20804%20searchable.pdf
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 the Air Navigation Order, Section 2 of Part B of Schedule 7, appropriate for a Microlight aeroplane class rating. Refer to Section 5, Part A, Subpart 2, paragraph 3.11-3.13.
3.11.2 Microlight pilots wishing to convert between weightshift and 3-axis microlight control
systems, or to a microlight with more than one engine, shall undertake differences
training given by a flight instructor entitled to instruct on the microlight aeroplane on
which instruction is being given. The training must be completed, recorded in the pilot’s
personal flying log book and endorsed and signed by the instructor conducting the
differences training.
3.13 Exercising the privileges on Microlight aeroplanes on the basis of an SEP(Land) Class Rating
The holder of a UK issued licence or any Part-FCL licence with an SEP rating, may,
subject to differences training on the appropriate class with a suitably qualified instructor,
exercise the privileges of their licence on microlight aircraft. However, any experience
gained in microlight aircraft cannot be counted towards the flying experience necessary
to maintain the full SEP or TMG privileges.
Essentially - all you would need is training by an instructor, and then that instructor to sign your logbook to say you've had the training.