I went and purchased a Ximeta drive and it works as expected
I initally connected the drive via USB locally & then partitioned the drive & used SuperDuper to backup for the first time via the USB connection. (so it wouldn't take as long to backup 100GB).
Then I installed the Ximeta drivers, plugged the drive into the network & mounted the drives wirelessly, ran SuperDuper again as a smart update & hey presto all changes copied.
The Mac software from Ximeta is OK, but nothing flash. You can't have more than one computer connected to the drive, so need to unmount the drive after each use.