The Castle nut gave me enough threads to hold the inserts and thread onto the end of the bolt.
I bored into the bolt end just far enough for the 'starter' part of the tap to occupy, so that I can get complete threads in the insert.
The end of the bolt is 'squared' so that the insert is held 'square'. I just thread the insert into the nut and lightly 'bind' it in place. Then I just clamped it in the mill vise and used the mill chuck to start the tap accurately, and we're off and running!