Hi, @david_barker 
To supplement the other good answers in this topic: the man page for the tune2fs command explains the purpose of the reserved blocks in an ext2/ext3/ext4 filesystem, in the section about the -m switch that @pavlos_kairis has rightly mentioned in his reply:
$ man tune2fs | grep '\ -m ' -A 4
-m reserved-blocks-percentage
Set the percentage of the file system which may only be allocated by privileged processes. Reserving some number of file system blocks for use by privileged processes is done to avoid file system fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the file system. Normally, the default percentage of reserved blocks is 5%.
I've found a similar discussion that took place here in the "Ubuntu MATE Community" in 2020 that seems relevant and may also be interesting to you: