I am suffering occasional gearbox problems with my Vauxhall Astra. I only bought the car a few months ago, but it is making grinding noises when I select reverse. I took it back to the dealer and it was wrongly diagnosed as a lazy clutch slave cylinder, which was replaced under six month warranty last week, but the problem still exists.

The handbook says that the driver should wait five seconds or so after depressing the clutch before selecting reverse. However, this still hasn't fixed the problem. What should I do?