JB1
I recently did what you propose, and I don't recognise the advice you've been given here. Haven't recently checked LASORS, but....
I went from NPPL microlight to NPPL microlight AND SSEA (Simple Single Engine Aircraft) in 14 hours of Cessna 150 time.
From memory, I was required to do 'such training as deemed necessary by my instructor', a period of instrument appreciation flying, and the tests, Nav and Skills.
The discretionary training was, I believe, a minimum of 3 hours, along with the other bits.
LASORS will tell you how to go from NPPL to JAR-PPL, but ALL your SSEA time will count.
Hope that helps.
CG