Thanks for the info.
Okay, I've run the Live Data View and checked the External Temp value. It remained constant for the 10/15 minutes or so that I watched. However, the '1hour Delta' value started at -44.2, quickly changed to 1.2, changed to 0 and stayed there for a while, then changed to 1.1 and now it is -29.5. I don't know what this value relates to? The status is 'Valid' for all items.
On looking at the graph produced for the last 12 hours or so it still shows the very, very zig-zag fluctuation pattern as before. However, for the time I've had the base station plugged in to the computer and running WeatherCat, it seems to be stable.
Most odd! I've not tried a clean install yet but may try that soon.
Cheers