Just an update on this:
We managed to get it fitted, all bled through and working perfectly this weekend. We used the smaller of the two o-rings to start with and also set the lever to the furthest span point to give a full stroke when bleeding the system. We bled at both the slave cylinder and the master cylinder just to be sure.
It all bled through fine and was working without problem, so we then reset the span adjuster on the lever to suit and off we went for a hoon.
Thanks for all your pointers.