As others have said it's really hard to advise without knowing your budget.
The best would be something along the lines of a 400 f/2.8 which is a good few thousand pounds. The downside is it's big, heavy and zooming has to be done with your feet.
Then you have the 100-400IS L USM, this is very flexible and my personal lens of choice.
Second hand you can find them for £750 upwards.
However I had some great results with the 70-300mm IS USM, absolute cracking lens for the money. You could pick one of these up
second hand for a little under £300. thanks roly great pics. i go to isle of man races every year and looking forward to giving it a go
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