So, I have made some changes. I wasn't convinced that the extrusion on the gantry was going to be rigid, so that has been dropped in favour of some solid lumps of metal. It is also much shorter, and the rails have been moved up to the top.

Click image for larger version.  Name: gantry rear.png  Views: 1526  Size: 280.1 KB  ID: 20926

Click image for larger version.  Name: gantry perspective.png  Views: 1502  Size: 216.2 KB  ID: 20930

What I'm wondering is if there is an optimum distance between carriages for the gantry.

Click image for larger version.  Name: how close can they go.png  Views: 1500  Size: 286.3 KB  ID: 20927

I dont have any worksop equipment beyond the basic tools, so don't want to drill into the extrusions or anything, but I'm getting a bit lathered about the hseer size of the machine, given that it's only going to have a work area of 600 x 900. Currently the overall dimensions are roughly 1400mm x 940mm. I can trim a small amount off the length, but is there any way of getting this more compact?

Click image for larger version.  Name: full assembly 1.png  Views: 1558  Size: 788.1 KB  ID: 20928

Click image for larger version.  Name: full assembly 2.png  Views: 1555  Size: 683.7 KB  ID: 20929