Just an update, and thanks to everyone, especially Richard whose measurements were very helpful.
The key blanks arrived today, and after a bit of experimenting to work out how to hold the blank in the mill (two 2" toolmakers clamps held in the vice was the final solution), the keys were cut.
I left the first a bit (0.2mm) wide, not realising that that was as important as the cut depth, but a bit of judicious filing sorted that. The other two worked straight off the mill.
Thanks again for the help. Some of the posts were most enlightening.