I have finally persuaded, by dint of actually reading the manual, managed to use the Polar G3 GPS unit to report speed/distance and the route to the CS600X. Unfortunately, I'm not exactly sure what made the difference. I think it was switching the GPS on before starting to record the exercise. What with all the faffing around, I ended up only recording a few miles on a ride curtailed by rain.
The GPS unit only needs to be configured (or "introduced") to the main unit once. You also need to select "Other" in the bike settings menu (oddly, you can't ever seem to deselect "Bike 1"). To use the device, you're supposed to strap it to your upper arm but I clipped it to one of the cables, so it hung a little in front of the handlebars. You turn on the GPS by pressing its only button, after a bit of flashing of lights it seems to find the satellites. But not before the computer's asked one to check the speed and power sensors (neither of which was actually fitted).
In use, I was comparing the reading to a conventional wired SigmaSport computer. The speed readings seemed pretty comparable, but there was a definite lag to the GPS derived reading compared with the SigmaSport. So, the jury's still out as to whether the Polar G3 GPS is realistically usable as a substitute for a speed sensor - I had hoped that forking out for the GPS would mean I could avoid having to buy speed sensors for the bike fleet. Time will tell, I dare say.