Used an A2Z disc brake adaptor... but had to modify the front plate slightly to fit the cro-mo rear dropouts. A2Z is designed for the larger flat aluminium dropouts. A bit of filing with a hand file and it fits!
Updates - finally managed to get an alloy rear end! Back to V-brakes on the rear. Swapped forks a while ago to a shorter 120mm Manitou R7. 100mm would be better but this bike gets the left over parts from the others!