X- and Z-axes both seem to be surprisingly stable and wobble-free (except for a slightly bent Z screw which I'll attempt to straighten later).

However, the Y-axis has a distinct wobble, so straight edges aligned with Y end up slightly wavy. The fault seems to be the 'fixed' pulley arrangement in the front two corners. Here, each pulley is pressed onto a plastic (?) centre with a hexagonal hole in it (same design for the M3D, but looks like square hole*). This transmits drive from the Y-axis spindles (hex bar), but is free to slide in Z axis. The problem seems to be that the hole is not central, so the pulley precesses as it rotates, pulling the whole X-axis assembly side-to-side with it.

It doesn't look like you can buy spares, so I might have to make something... stay tuned!

*it's a bit like this spare part on the M3D website, although this is meant for the bottom end of the drive spindle, not part of the Y-axis gantry.