Hi Rab -
1) For that size spindle I would use 20mm cars vs 15mm on the Z. The bottom cars on the Z take a lot of beatings and need to be strong. Especially if you want to take full advantage of 2.2Kw. oK for a 1.5Kw
2) The base screw bearings mounted on the cantilevered flat plate is a candidate for vibrating. Either mount it lower or web it some how
3) The large N34 Z motor again is on a cantilever and is a candidate for vibrating stiffen it or support it better
4) the construction extrusion bolted to the back of the gantry is way too inefficient. Since your cutting up plate make a plate box gantry way better
5) Its not ideal chopping so much metal out of the Z plate for the drive nut. Its usual and better to do the trench in the gantry so the tool plate is as stiff as can be
6) The bearings behind the spindle need to be further apart the 2.2kW spindle if used to its max will generate a lot of moment on this area. Especially since you have picked 5mm pitch for the screws, these will move small trucks quite easily so forces will be large
7) Its coming along... Peter