Well, I've just flashed the firmware up to Version 5.21 and done all the usual stuff associated with that process. All went well - so now we wait for some clear sky. Because it's a new moon in a few days, that probably won't happen of course.
If that doesn't fix it, I'll take that advice and give Don a call. I bought it from Bintel so I expect they'll be happy enough to help.
EDIT: Well that didn't fix anything. Damn.
Peter