Bingo!
Found signs of weight added beneath the film, cut in, drilled and removed about 1 oz!
I then found a heavy prop nut (came off a foamie Spitfire electric model), approx 1 oz in weight. Weighed and only needed 15grams to get the balance point. total weight 48.75 ounces. Huzzah!
I am not sure about using the heavy prop nut as the motor is rear mount and I think I remember reading that there can be problems with bearings?
Anyway, if I stick to weight on the motor mount /bulkhead instead I should still bring it in at 50oz or a bit under
Thanks