Great, thanks for that, clarifies a lot and answers a couple of questions I hadn’t realised to ask yet 
The CAD files are super helpful to figure out what’s going on and to have a solid place to start, unfortunately Fusion 360 cannot change the display units in an existing file apparently so, despite many of the measurements being input in mm it will only show them in inches. I’m cloning them into mm to work from.
I’m just trying the X at the moment to see if I’m OK with the remaining slack after fixing the worst offender. We’ll see how long I can go before the Y beams get railed. Once I’ve done the X I may feel more comfortable cutting the more complex parts for the Y.
And I’m with you on the achievable precision thing, if I want microns I need a mill, not a CNC router.