I've managed to sort the problem, by going back to an old restore point, when the track pad was working, and the "device select" tab was still in the mouse properties window. Then I undid that restoration, and the track pad continues to work, along with the device select tab remaining. But I'm wondering why this happened in the first place, and how.
I had installed Media Player 9 and real player free version, along with some fiddling that the office IT guys had apparently done to the network connectin - presumably these somehow clashed with the settings. But what I don't understand is how going back to an old restore point, and then undoing that restoration, effectively changes the settings - kind of implies that undoing a restoration doesn't quite undo all the settings?
At one level, I'm pleased to no longer have the problem - but would still like to know why the solution worked!