I followed the advice you all gave me & checked / tightened / aligned what I could.
Retesting the x axis gave me the following figures that I'm pretty pleased with.
DRO Dial Gauge (all values in mm)
0 0
1.003 1
2.03 2
3.01 3
4.01 4
5.04 5
6.03 6
8.06 8
10.05 10
I cut some test holes, a rectangular cutout and some circular cutouts. Both were within 0.05mm (~2 thou?).
I changed the cutting bit to a 2 flute 1/8" dremel router bit and got a much much cleaner cut with little to no melting of the plastic and little burr left on the edge.
Some pics here of rectangular cutout & circular cutouts:
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Thanks for you help, you can be assured there will be more questions to come!