Indeed.. The discrepancy was a very slight misalignment of the original two curves on which the cone is based. This meant that I had to start from scratch, which is just as well because the first version was not accurate according to photos I just found. This model of turret was only on the original batch of Stirling bombers, and as such not all that well documented.

I am not sure whether this new version is any more accurate, but at least it is smoother!