Ah - light bulb moment - take the springs out!
I got one of those ebay/Woodford Tooling jigs - for an Axminster AW106PT2. Not sure if it is identical to the Axminster one, or a cheap copy, but all I proved initially was that the spring pressure was enough to push the central body of the jig up out of the housing. They are not exactly well built.
Mind you, there are no instructions either so I was groping in the dark a bit. Neither does the Axminster manual mention removing the springs.
I've taken the springs out and had another go but I still seem to have to use a straight edge on the outfield table as a best setting guide. I don't find the jigs very repeatable but I can use them to move the knives up and down and hold them in position as I tighten the screws.