RouterCNC - thanks for taking the time to explain all that, I appreciate it. I've had a bit of a play with your stiffness calculator spreadsheet and I'm getting gantry deflection of <0.05mm as I would expect following Jazz's advice.

I think I'm going to redo my frame design using 60x60x5 for the legs, 120x60x5 for the rail supports and maybe use 50x50x4 for the bed and triangulation / stiffeners.