Thanks for your help - I did contact Eloy, and he confirmed that the files in there are not important and can be deleted (as long as no backups are in progress during the deletion).
What I think happened was that a course
backup (of a huge course) had gone wrong - possibly interrupted several
times in the middle. This had put a lot of files in moodledata/temp, making my own backup huge (I do my own backup).
The problem seems to have gone away now.
Hardware and performance
moodledata/temp/backup very large
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