Thanks, both, for the responses.
The link provided by sully666 gave me the information I needed. Simple when you know how!
On Windows 7, either of \\192.168.1.254 or File://192.168.1.254, entered into the address/path area at the top of the Windows Explorer window, was sufficient to browse to the contents of the USB stick attached to the router.
I see that the File://192.168.1.254 option was mentioned in the BT article in the context of using the Windows Start button to "Search programs and files", though it also worked when entered into Windows Explorer.
Right-clicking on the drive, if I'm recalling correctly, as displayed in the lefthand panel of Windows Explorer, gave me the option of mapping the USB stick to a drive letter. Having done that, it is listed in the lefthand panel of Windows Explorer under Computer, as a drive letter (as well as in the righthand panel of Windows Explorer as a Network Location). Having mapped the USB stick to a drive letter, as an alternative to finding the drive among the entries in the lefthand panel of Windows Explorer and double-clicking on it, I can simply enter the relevant drive letter in the address/path area at the top of Windows Explorer.
Interesting that, when I previously followed instructions to use a USB stick plugged into a Talk Talk router, the instructions recommended restricting access to the USB stick with a user name and password, something which is not suggested by the BT help page. Wonder whether Talk Talk give credence to the possibility of a local network drive being accessible to mischief makers on the Internet. Not sure whether setting up a user name and password for the USB stick is a wise precaution or something for the paranoid. Of course, what's wise and what's paranoid may depend on the nature of the content which could potentially be exposed. Aside from the possibility of restricting access to the USB stick, I also have the option of using encryption software which came with it. I think I recall seeing one or more complaints about vaults encrypted with that software becoming corrupt and material being lost. Can try it for myself, I guess, whilst trying to remember never to trust the USB stick - and, more so, never to trust an encrypted vault on the USB stick - with the only copy of anything. If the USB stick did become accessible from the Internet, the risks would not necessarily be confined to the content on the USB stick being read. I wouldn't want to allow strangers to write to it and, potentially, cause me to host unpleasant content. Is the networked-attached storage more exposed to the outside world than, say, my C: drive, I wonder. (Of course, I use a firewall. Very occasionally, I get a report that it is turned off and am prompted to turn it on - and I'm not sure whether it provides protection from the moment the computer is switched on; there's a lot which goes on when it starts up and the process is not quick!)
Also interesting that I changed the name of the USB stick in the router's advanced settings but, so far as the network is concerned, the USB stick is usb1, which matches neither the original name which was shown in the router's advanced settings nor the new name I chose.
im sure when I clicked on network in windows 7 it was showing just as another network PC by default under the name 'PlusnetHub1' or similar?