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Oh bloody hell. They *are* work trousers - as in the bottom half of a suit! :-/
Apparently it's MY fault, for asking him to go there on his way home from work.. hence me having to try and remove it.
I'm really grateful for the suggestions, but I'm a bit nervous of trying them and making it worse. I thought you'd just recommend something like Vanish, and it would just go.
Ok, what if I took them to a dry cleaners. Would they be able to get it out, or is oil really difficult to remove?