This is NOT a rating of the service I got from the Lock Shop; they were very quick to get the order to me and I found them very good to deal with.
It turns out that the unit I was trying to replace was an older one, and Legge in their "wisdom" have changed the position of the tie bolt just enough to force you to buy both halves or you will never get them to go together. There doesn't appear to be any reason for this other than wanting to sell more units. Not impressed!