ok, first attempt using sketchup 8

[JAZZCNC]

Jazz,
Is the plate with the slot still ali or wood, i'm confused with cutting with a circular saw.

Yes it's Ali plate and yes it cuts very easy using hand held circular saw used for cutting wood so long as it has a TCT tipped blade, preferably with a thin curf.
There are purpose metal cutting blades as well but can be expensive and not required if just doing the odd bit of Ali cutting.
You could use a jigsaw but the hand held makes quick work of it and cuts a nice straight line.(Better if you use a guide or piece of wood to run against)
The hand held will cut 10mm plate in one pass just lower the blade in nice and slow and push at a slow feed. Make sure you use goggles and long length sleeves because the chips burn like crazy.

You can even cut Ali with hand held router if you take shallow depth pass's. Thats how my first machines gantry sides and Z axis where cut, I made a MDF template then used TC 6mm straight top bearing guided wood bit and cut around the templates in 0.25 pass's.!! . . . Scarey shit at first and took awhile to do but it worked ok.

If i did beef the gantry up this way would i then be able to cut anything other than light wood, would it be able to mill aluminium if required

With this setup you'll be ok for anywoods really so long as your not expecting to cut 20mm in one pass.!! and my comment about light wood use was aimed at your first design mostly.
Regards cutting Aluminium then yes it will but you'll need to be sensible, don't expect it to take 2-3mm depth pass's or even 1-2mm. You'll be in 0.25-0.75mm Max area for acceptable results. That said this is still usable thou and if your realistic with DOC/FEED rates etc then it'll be ok.