Sorry for the slow reply.
There is no Recycle Bin active. This was the first thing I checked.
Windows and the NAS both report pretty much the same disk usage. A few MB difference.
Firmware is 1.10. I can't get to the back of the NAS to check the hardware version at the moment. I'll try later.
I'm going to try emptying a disk, as I have a big enough USB drive to move it, and see what the file sizes say then.
In the case of the Linux has an overhead, this shouldn't result in the free space on the device being reported wrong surely? Wouldn't it just effect the reported capacities?