Thanks Routalot.
I agree that external triangulation would pretty much solve the problem, at least in the locations that have base support that can be extended. The issue is with my ball screw assembly, it will prevent any bracing from being mounted more than a third up the C channel beam, and most of the deflection is coming from within the channel (the c displacing up in between the bulkheads).
I will try modeling a complete closure plate for the sides (essentially making it a rectangular box section), I think that will stop the localised displacement of the C flanges, and wont cost all that much in 6mm plate.
I think the machine will still be pretty stiff (even at 37N/um in this direction) but it's not in keeping with the rest of the build, and the way spring constants stack, one weak component has a pretty dramatic effect on the entire assembly.
EDIT:
I modeled closure plates on the full length of the x axis beams, and it essentially doubled the stiffness... its not the 5x I wanted, but it will be well worth the money I feel, and it looks much better.