Okay, I appear to have cracked it, the problem that is, not the cab !
With the combined efforts of myself and a modeller friend we discovered that the trick is to remove the radiator grille to reveal a locating lug behind it, after much fiddling around the lug was then depressed with the aid of a 6 inch steel rule, depressing this lug first (which isn't easy, it's quite a strong lug) and then releasing the bottom rear of the cab has the desired results, once the cab was off it was a matter of more fiddling around, gentle pressing, pulling & levering to remove the windscreen, the cab and chassis now lie in bits awaiting the attention of workshops (once we decide which of the 4 current options we're going to turn the thing into!) so it may take a while before any further progress is made.