I think I've solved the problem
I think the issue may stem from the fact that the ISS was disconnected yesterday when I had to do a repair to the ISS circuit board This meant that there was an hour of missing ISS data, though the indoor data was recorded fine. When I downloaded data to my PC today before adding any sensors or making any config changes I noticed that in addition to an updated (larger) 2021-09.wlk file, there was also a 2021-12.WLK file which was showing as only 1k in size.
I'm not sure what caused the 2021-12.WLK file to be generated, I'm guessing that the ISS going down may be linked, but don't know for sure.
When I tried to import data after fitting the sensors the Weatherlink software message displayed that it was importing 513 records, i.e. the total capacity of the datalogger, even though there was less than an hour of new data to import. I now think the existence of the 2021-12.WLK was possibly causing this problem and confusing the Weatherlink software.
I decided to investigate the 2021-12.WLK file more closely using an Excel macro I've written to import header info from WLK files. It turned out that the WLK file was just a header index with no actual data, just 212 bytes in size. I decided to delete this file - I have copies of all my WLK files in another folder just in case of data corruption.
When I went back and tried to import again, the download was just 6 records rather than 513 and there is now plottable solar and UV data. I now need to tweak my macros to import this into Excel