Have you factored in any flash? I ask as I have a Canon 100mm/2.8 macro. I always thought the macro results were a little ............. dull. It wasn't until I got a 430EX II flash and velcroed it to the hood of the lens, that my macro photography changed. It was much more vibrant and well worth every penny. I would definately suggest getting a flash as well. If you got a Sigma 105mm, Tamron 90mm or Canon 100mm for example, you will be able to get flash too within your £500 budget.
I hope that helps.