OK so I've figured out enough of Fusion 360 to model something - not going to win any awards but at least it's clearer and I can edit the design as needed.

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The blocks on top of the X carriages are set to 40mm thick currently
All rails are 1000mm , to be adjusted if needed
Aluminium profile on gantry is 120 x 40 with 90x90 on top.
The purpose of the ballscrew and ballnut running inside gantry plate is to allow an L angle (or perhaps long brush) to be fixed to the extrusion above the screw and cover the ballscrew from dust/chips
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Issues I'm thinking of so far is the the Z axis clearance isn't huge, 125mm from top of baseboard to bottom of gantry, although probably OK for my needs.
The Z design with rail on front and top is based on the fact Z clearance would be even lower if I put rails on top/bottom of profile. I plan to add extra C shaped plates to side of Z axis to strengthen - will it be enough/OK design?
Y axis travel is limited by how far forward Z is - losing about 275mm so probably need to bring the carriages closer together and gantry plates narrower, maybe mount motor to back of gantry extrusion instead with pulley.
Any comments on those bits? thanks

Aluminium profile arriving tomorrow - only thing I've ordered so far, but will be good to get a feel for size. Also built a 1.2m square workbench for it , 25mm ply top and 50x100 wooden frame on castors, hopefully that'll be rigid enough.

Ryan