It sounds like the one you are talking about is the second pattern, which continued in production till about 1952. Then the Denison reverted to woollen cuffs, but you can tell these later ones from the first pattern by the rear studs to hold the flap up and the vertical centre seam at the front. The 1959 pattern introduced the full zip; the earliest date I've seen for these is 1960 and I have seen a 1959 dated half zip example, so I imagine the changeover took effect in 1960.
Postwar Denisons are still quite pricey. I would say a decent example of the type you are talking about should cost about £300, maybe a bit more. Essential to see pictures though.