Thanks, mixture; you confirmed what I suspected in that my ancient, decomissioned-until-the-last-few-days, four port ADSL router is a switch not a hub.
Please will you be kind enough to elaborate on your suggestion for an alternative way of me sniffing everything to and from the NAS. As well as the old router I just described I also have a brand new Netgear GS605 gigabit switch but of course it is of unmanaged type.
TVM!
PS - I have two gigabit LAN ports in the PC I will run the sniff from but believe they relate to one physical card. (I need to check since I didn't pay any attention at the time to that part of the spec as having two ports was a Brucie Bonus I didn't need)