Thank you for your suggestions and input - problem now solved!
No results from trying the window switches with the engine on or off. Looking on YouTube, I stumbled across a video of someone with a Fit that had the LHD drivers window unable to close properly. He demonstrated a "re-programming" of the window electrics using a sequence that featured the key in position 2 and then off, with the window operating switches on and off. The sequence he used had to be repeated four times.
With nothing to lose, I tried this, and the window is now working as it should. The channels have now been treated to a silicone spraying, as suggested (all windows, not just the problem one).
Once again, thank you.