A Supreme Court case has judged on a penalty for breach of covenant in a multi million pound company sale, and on an £85 parking fine. It concluded that both are enforceable, as they don't impose a detriment "out of all proportion to any legitimate interest of the innocent party." So don't assume that just because a penalty of this type is more than the actual loss suffered, it won't be enforceable. And this can effect huge deals and the man in the street too!