Many thanks for the reply. I actually had a go at this tonight with a friend or 3 and as luck would have it we actually did pretty much the same thing as you.
Measured the hull and set the rear rollers where we thought they should go, we actually ended up moving them in about 15mm each as when we first tried to winch the boat on the keel just hit the swing beam before the rollers picked it up. Then set the winch post so it gave 25mm overhang at back of rear roller.
I have to add though that we didn't have a slip way so we were simulating a worst case scenario of a beach launch and winching the boat off the floor onto the trailer.
Then set front rollers to straddle chines and moved bow keel roller to take some weight.
As luck would have it when we fitted the new axle we guessed at its position and the tow hitch weight was almost spot on so we didn't need to move it.
Brake cables, flush kit and lighting board to fit tomorrow and that should be it!