I am being prosecuted for speeding but I think that the police evidence is inaccurate. Am I entitled to a copy of the evidence so that I can have its accuracy verified?

Unless you accepted a Fixed Penalty ticket when you were stopped, the answer is "yes" - See this link for a summary of the legal position.

This forum post covers the topic of disclosure, in more detail. Here is an example from the forums of a defendant's 'defence statement'.

The legal position is quite different in Scotland and this forum post has more information.