Very pleased to say that the replacement process was a success, although the air around the work area turned a dark blue early in the process and remained that way for some time...
I incorrectly stated that the screw was between the leaves, in actuality, it goes below them and provides tension to the lower leaf. Remove it prior to installation, after the spring is in, compress the leaves together and install the screw.
Definitely have a main spring vice if you are going to do this (I didn't, and used vicegrips with leather between them and the spring to avoid marking it). Even with the proper tool, I don't know that I'd recommend it "in the field". A small vice with padded jaws (to hold the lock steady) was a definite bonus. The process was much more difficult than replacing the mainspring on my Traditions flinter.
Calum