Having finished my router design. I posted some pictures in the forums at http://www.mycncuk.com/, asking for comments. A few of the guys there suggested that my design was potentially weak in that the Y axis SBR20 bearings were loaded incorrectly.
I've had a re-design of the Y gantry. This is now formed from 3 pieces of 15mm aluminium plate bolted together. This depth of 45mm is exactly enough to allow me to mount the Y SBR20 rails on the top and the bottom of this section. Also, I've changed the Y stage to be a rectangular box that surrounds the Y gantry, with the ballnut on one side, and the Z axis on the other.