Thanks Eric. I've never used exiftool before, but it's certainly a wonderfully useful tool.
I think I have solved (or at least identified) the problem, but Lightroom is still showing rubbish in the Lens field, so I wonder if I can pick your brains about this.
After a little probing with exiftool it seems that the problem lies with the LensModel tag in exif. Images taken before I swapped the lens on and off show a LensModel description of "EF17-40mm f/4L USM". Those after show a tag of "???????????????????????????????????????????????????????????????". I assume (perhaps wrongly) that it is this string of question marks which leads to the ÇÉÉÉÉ... entry in the Lens field in Lightroom (see the attached image).
Anyway, I therefore use exiftool to edit the LensModel entry in all of the bad files, using this command line: exiftool -LensModel="EF17-40mm f/4L USM" /Pictures/badexif.
The change was successful, but Lightroom still shows bad data in the Lens field, even though I have deleted the files, reimported them, re-read the metadata, optimised the catalog, everything I can think of.
Any idea what might be going on? Does the Lens field in Lightroom read the LensModel exif tag, or have I got it completely wrong?
Andy