Later in the day yesterday I stripped down the spin indexer and took the division plate off. I mounted it on the dividing head chuck of my Proxxon mini drill and reamed out all of the holes to 5mm. 

Then, having ensured that the locking collar was tight on the spin indexer and the Y table locked to ensure a consistent depth of cut, I had another go using the opposite end of my aluminium stub.

All went swimmingly and I ended up with a perfect gear.

Now I need to do it for real to produce the missing final drive gears for my two Shogun Gearboxes.