The job is done. Painstaking work it was too. Turns out it was only 346 files in the end.
Every single activity categorised properly and given the same name it had in Garmin Connect.
I have a big monitor which was useful (30"). I had GC open, SportTracks open, folder open for the files I exported from GC to "land" in and afterwards be imported into SportTracks from. Then it was a case of open an activity in GC, copy and paste the name and any notes, click export in GC, select tcx for export format, download it, hit import in SportTracks, select the new file from GC, name it, categorise it, get notes in, and then on to the next.
Mind numbing stuff but I'm glad it's done