I got a local repair man to look at it and he found that several years of my big hands assaulting the computer had squished some of the elements within the touchpad to death.
Apparantly the earlier Acers (mine is about 4 years old) had notoriously unreliable and unstable touchpads.
Fortunately he had a relatively new laptop in that had been badly damaged but the touchpad was OK, I got it for nothing and fitted for £10.