Okay, Long term- I will probably rebuild the Z axis and most of the gantry. Problem is, I cant really mill aluminium at the moment. too much flex in x-axis. Heres what im thinking of doing to get it a bit more rigid.

Add two 15mm linear rails (4 carriages), to a 120x530x20mm aluminium flatbar backing plate (fixed to gantry). A 2020 extrusion top and bottom to aid fitting.

Here is the before and proposed after...

Click image for larger version.  Name: gantry 2.jpg  Views: 337  Size: 93.5 KB  ID: 23522

I'm worried about the flex in the 2020 extrusion. Anyone got a better idea?

Thanks.