Well, I have now found half the solution!
Connect the NDAS device to one of the ports on the "white box" router so that it is on the same side of that as are the two PC's when they are connected by cable ethernet! Then both PC's can discover the device during the s/w installation process (which has to be done separately on both machines) and can access it.
Now here is the unsolved part of the puzzle:
If I then move the NDAS box connection back to the WL 500G, then one of the PCs' - a laptop with both cable and 802g connections - briefly re-recognized the box but lost the connection when the server was rebooted.
However, I then moved the NDAS box back to the white box router connection, and disconnected the ethernet cable to the laptop - and the laptop can still see the NDAS box via its 802g connection to the WL 500!
So, when the NDAS box is connected to the white box router, the laptop (and presumably any other device connected to the WL 500g) can see it via its 802g connection to the WL 500, - but any PC connected directly to the white box router cannot see the NDAS box when it is connected directly to the WL500g.
The white box router is unmanaged, so I can't do anything about its setup - so I think I am stuck for the time being, but at least it now all works after a fashion!
Any further thoughts from anybody would be appreciated!
Regards
John