Thanks Doc, I've set up a drilling jig with one of those sliding drill holders and a mounting frame to try and get it just right @90 degrees. I'm going to set up the neck, and do some re-measure so that it will be worked from the final neck position.
I'm pretty confident that its good, but I am going to do some test drills in spare ash just to verify squareness etc. and see how we go.

Last thing I want to do is cock up the scale length as that was one of the few things I got right first time around!