>the single 15mm Y axis (or I think you call it the X axis) plate could do with stiffening as I don't think it will resist twisting very well, especially if the Z axis is at max travel.

Do you not think that the plate that it's bolted to (which sits horizontally behind it) would be enough? When the Z is at max this is the part that's going to be taking the brunt of the forces - hmm... Will have a think.

Cheers.

w./