As a glider pilot I benefitted from the max allowed 10 hour reduction in the PPL syllabus so still had to do 35 hours of training before the flight test. I can't remember what allowances were given to NPPL(A) holders seeking an ICAO PPL but but I'm pretty sure they had to do more than 15 hours additional training & supervised solo.
We need to go back a mere 20 - 22 years from now, before the period to which you refer. I can't remember that the minimum hours were demanded then other than the obvious differences were detailed including; 2 hours stall/spin awareness and 1 hour instrument flying as an example. A Silver 'C' could complete within 12 - 15 hours and Bronze 18 - 20 hours. The NPPL when it was introduced post JAA but before EASA was a truncated PPL and only required the differences to be completed to upgrade to PPL. I'm sure that BEagle and Whopity can put the meat on the bones.