Thanks for the reply Ray.
Since you mentioned the firmware it occurred to me that maybe I should install the version V2 ROM that I bought some time ago but had not got round to fitting.
I've done this now and the V2 ASCOM driver is now reading Astro-Physics V2 Driver: v.5.30.10, 1200GTO GTOCP3, Firmware: V2
However, the slewing behaviour is still the same: in the driver, a move rate of 12x gives a sensible result, but 1x and slower are much faster than 12x.
This problem didn't start with the latest version of the ASCOM driver. It started randomly. This makes me think it is more likely to be a hardware issue.