Have measured both the old and new cables. Distance between the new Outer cable and inner cable is 9mm too long. So added an additional ferrule as an adaptor (cut down to 9mm and drilled out so that the inner cable will pass through it). When adjusted the bike idles perfectly now !
So many thanks for your advice.
I will either araldite the additional ferrule into the adjustor, or ask a friend with a lathe to make a small adaptor.
However, first I’m going for a ride tomorrow to the Classic Bike Show at Stafford Show ground.I should have bought a MZ years ago, what fantastic value for money, I’m really enjoying fiddling with it and riding it.