Thanks for the replies.
This is an install of MC32, so cannot have been that long ago, even if enabled on install.
As I didn't know where the logging menu was, I cannot have enabled this knowingly, nor altered the default limit to 100s of GBs.
How was MC able to recreate the 90GB file instantly after I deleted it and emptied the trash?
This was my server running 32.0.8, as a test I re-enabled logging and checked how the log.txt file grew, with MC just sitting there.
After 30 seconds it was 250MB, after 2 minutes 30 seconds it was 2GB.
Feeling this was not right I updated to the latest 32.0.16 and rechecked.
After 2 min 30secs it is now only 600k rather than 2GB, which seems more reasonable.
Don't know if it was the update or just the restart that changed its behaviour.
Anyway, it is gone now thanks.