Hi suealpha,
I think you could indeed be right about your idea that 50 or 60 miles south of you could make all the differance, the Medlar (mespilus germanica) originates from SE Europe and C Asia but has long been cultivated in this country.
However there is a variety which has been selected for its fruit-bearing potential, which is less thorny than the wild trees and it also has considerably larger leaves, which is a named variety known as 'Nottingham' so maybe its this one ? All the Best Tbird+