Image of File Explorer as promised. I cannot assign drive letters to these partitions in Disk Management. I guess the conclusion to be drawn is that Disk Management does not recognise non-Windows...

Yes, I am sure. I have the root partition on this drive, which is an ssd drive. The other hdd drive contains data partitions. I don't know why it's not showing up in disk management, but it is...

I see. The figures do add up as you have specified, but I think it is confusing that it reports the capacity of the other partition as though it was part of C: The 139.70Gb partition is an EXT4...

If I open Settings in Windows 10, then go to System > Storage, I get the following usage for C:
161Gb used of 285Gb
whereas File Explorer gives the more sensible 21Gb used of 145Gb, which is almost...