Having spent a couple of hours yesterday pratting about with the PID controllers for the X and Z axes, I seem to have arrived at something approximating to the best I can expect from my setup. It may not be stunningly good but it's a lot better than I'd ever achieve when using the machine in manual mode.
Ideally I'd have tuned the servo drives on their own before tuning the outer position loop within LinuxCNC. However, the "software" provided by Lichuan is almost unusable. For a start it won't even run on most of my Windows 10 machines. Finally, having installed it on 4 of them, I have a laptop that will both actually run it - and have it speaking English to me. Whoopeee shit. Another 2 hours of my life I will never get back, as they say.
Here's the link to download page where you download said "software", along with the "user manual". The "detailed" version of the manual is only available in Chinese but contains little more than the "simplified" version.
I then needed to download the driver for the USB-RS485 device before I could connect to the servo drive through a serial port (COM3) using a hacked ethernet cable:
This revealed the "software" in all its glory. It's just a table of parameters that you download from the drive. You can change the values in the light blue column and then upload it back to the drive. That's it.