However, I thought I'd give Logitech a ring. I got through to a friendly, knowledgeable chap called Michal who, after taking a few details and asking me a few questions about the problem, confirmed that the mouse was likely to be kaput!
He apologised that the model I had was no longer made and offered me a new Performance Mouse MX (£90).
Bearing in mind that I only paid about £30 for the previous mouse I thought that Logitech had gone above and beyond the call of duty. They are not stupid and know that by impressing me, I will most likely tell my friends what a good company they are - and they are correct. I am telling anyone who will listen that Logitech are fantastic. I have always loved their webcams and mice but until now had never had need to test their customer service. I have to say I am very impressed with them.