... you can fly a single seat 3 axis microlight as an unregulated aircraft in the uk without a licence
No, you can't. SSDR refers to the certification status of the aircraft, the pilot still needs to be licensed.
does my current 3 axis experience (about 40 hrs/year) with my French microlight licence count as 'logbook evidence'?
Probably? ANO Schedule 8, Part 2, Chapt 2
(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.