I'm worried about something stupid I did at the Council tip today!?

Such high morals are to be admired! There is no need to call and confess. If you are found out, make abject apologies and be prepared to face the consequences--though it is extremely doubtful they will go to the bother of a manhunt for such a small infraction. After all, it will be a very simple thing to move the bin liner to another pile. I think it's safe to say you can rest easy.