Thanks very much for the responses guys, they're much appreciated. Here's the offending lines:
Yep, cleaning vertically is also a bad idea.
The hell of it was that I felt that I was being very gentle cleaning the sensor, but if I managed to get a nice hard bit of grit under the pad, and the first surface is plastic, then I can see how easy it would be to get a scratch.
Anyhoo, I reckon I'll get myself a big ol' bag of supplies and give it a good clean. After all, what have I got to lose, right?
Thanks again folks,
Rob