Untangling the spaghetti strands of wires behind my desk I managed to isolate the cord for the existing mouse's controller - but it wasn't until I traced it back to the pc that I remembered it also controls my cordless keyboard. Scratch unplugging that for an idea.... Where it reached the pc it split into two different ports. By the process of elimination (50/50 didn't take long) I worked out which of the two ports housed the mouse cord, unplugged that plug and left it dangling.
Luckily new mouse controller has USB plug so it was simple enough to plug in, and it worked straight away. So now I have two controllers littering my desk, along with the wireless router for the laptop, the modem, my cordless phone... running out of usable space here.
And as a footnote to the above - scroll wheel hasn't worked for so long that I find myself automatically going to the scroll bar on the side of the page.