Bill wrote:
Not sure if anyone has tried this before, but here goes...

I have an existing CAT5 cable running between the downstairs and
upstairs of my house, length around 10 metres. Each end terminates on
a wallbox.

I'm now in a position where I would like to connect a downstairs USB
device (my Topfield PVR) to the PC upstairs.

The run length is too long for USB reliably, so I am considering a USB
CAT5 extender. I could run in another CAT5 cable for this, but it
would be a bit of a pain. HOWEVER, 10/100 BaseT network connections
only use 2 of the 4 available twisted pairs, hence I have 2 spare

The cable doesn't make a difference, you don't get any greater distance
by going to CAT5.
Obtain a hub.
Plug it into the middle of the run, on two 5m cables.
Job done.