I have been finding that on several pcs that I use, the free space on the C drive has been reducing constantly over time.

The existing tools within Windows 10 didn't help as they don't make it easy to find out where the large files and folders are, and the tools on the disk don't seem to cover clearing user/AppData/Local.

The free version of TreeSize (*) was very useful in identifying the problem: OneNote backup and cache folders.  (I use OneNote extensively, as a sort of personal StackOverflow for recording work done, problems encountered and how solved).

https://superuser.com/questions/1468754/onenote-reducing-storage-footprint-on-pc was very helpful in this regard, I was able to change the location of OneNote's cache and backup folders to a separate fixed drive within the pc.

Update * : I have just bought the paid for version as a thank you to the JamSoftware team for helping me identify the problem.