After a bit of measuring and figuring out how to mount the pulley, I decided to turn the shaft down to 12mm to match the pulley. This provides a shoulder between the 12mm pulley diameter and the 15mm bearing diameter so provides lateral location for the pulley. The pulley will be secured with the 2 grub screws onto flats on the shaft along with a 3mm cap screw.
The ballscrew pulley is from SDP/SI, part number A 6A55M050NF1512, and is a GT2 5mm pitch, 15mm wide 50 tooth aluminium no-flange pulley.
I removed the ballscrew from the mill and mounted it between centres in my lathe to turn the end of the shaft. This was sized to provide an interference fit to the pulley.