I had a similar dilemma when I got back in to GA a couple of years ago.
There were two ways around it: first was to exercise the PPL privileges of the ATPL, and complete differences training onto the microlight - downside to this was none of the hours accrued flying the microlight, counted towards the PPL hours requirement after two years.
Other option, which the LAA approved, was to do training up to and including the skills test, then they would issue me with a NPPL. I took the second option as it'd been over 10 years since I flew light aircraft, and was probably the safest option too! Looking back, I think that was probably the most prudent way to do it.