We are now into a Mish Mash of UK and EASA regulations. Differences training for an EASA Class rating is covered in FCL.710
If the variant has not been flown within a period of 2 years following the differences training, further differences training or a proficiency check in that variant shall be required to maintain the privileges, except for types or variants within the single-engine piston and TMG class ratings.
Of course there is no Class Rating in a LAPL but the inference is that if your experience predates the licence or rating you have grandfather rights.
Art 150 requires:
(6) 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 Chapter 2 of Part 2 of Schedule 8, appropriate for a microlight aeroplane class rating.
Ch2 Part 2 States:
Microlight class rating
(1) Subject to paragraphs (2) to (6) and to the conditions of the licence in which it is included, a microlight class rating entitles the holder to act as pilot in command of any microlight aeroplane.
(2) If the current certificate of revalidation for the rating is endorsed “single seat only” the holder is only entitled to act as pilot in command of any single seat microlight aeroplane.
(3) If the aeroplane has—
(a) three axis controls and the holder’s previous training and experience has only been in an aeroplane with flexwing or weightshift controls;
(b) flexwing or weightshift controls and the holder’s previous training and experience has only been in an aeroplane with three axis controls; or
(c) more than one engine,
before exercising the privileges of the rating the holder must complete appropriate differences training.
(4) The differences training mentioned in paragraph (3) must be given by a flight instructor entitled to instruct on the aeroplane on which the training is being given, recorded in the holder’s personal flying logbook and endorsed and signed by the instructor conducting the training.
There is no mention of any time limit for this training so as long as you have log book evidence of having legally flown the variant then that should suffice.