This evening I put the finishing touches to my new cheap-and-cheerful network storage: a USB drive attached to my BT Home Hub 2.0 (the shiny, black one).
Step 1: USB drive
The first step was to buy a new USB flash drive. I went for this one from 7DayShop.com. It’s a 32GB USB 2.0 drive and cost me £20.99. Usefully the swivel cap comes off quite easily.

(When I tried this out at first I used an old 256 MB flash drive that I had in my Big Boy’s Drawer of Interesting Things™.)
Step 2: BT Home Hub 2.0
Round the back of the BT Home Hub 2.0 is a USB port. They’ve even, conveniently labelled it “USB”. Plug the USB drive into the port.

(The dust is optional.)
Step 3: Connect with Windows Explorer
Assuming that you’re connected to your BT Home Hub, open a Windows Explorer window and enter the following network address in the address bar: \\BTHUB\Disk_a1 then hit Enter.

Step 4: Map a network drive
To save you having to type in the network address every time you can map a network drive to that location.
In Windows 7, open My Computer and click on the “Map network drive” button on the toolbar at the top:

A dialog windows will pop-up. Select a drive letter and enter the network address, as before, in the Folder input box:

Then click Finish.
You now have a network drive:

Security
I’m going to use mine for backing up a few files and as a useful location for sharing documents between PC and laptop.
I imagine that this isn’t the most secure of solutions, as anyone with access to the network could gain access to the files, if they know the network address, but as a cheap and cheerful way to share files across multiple computers without the other PCs needing to be switched on this is ideal.
Update
Oddly, after a couple of weeks of this working fine I can no longer connect to \\BTHUB\Disk_a1, the PC just tells me that it cannot find the hostname.
It appears that this is not an exact science.