HELP! how do you get odour of cat pee out of barbecue cover?

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This post is related to my (other post about) problems with the neighbour's cat peeing on my covered barbecue and doing his business in my back garden. How can you get cat pee odour out of tarp/heavy plastic barbecue cover? We've washed with water and dishwashing detergent but it's not helping! GRRRRR!!!! :-{{