If all the facts which make the transaction unlawful were known to the parties, as I think they were, ignorance of the law will not excuse them: see Churchill v Walton ([1967] 1 All ER 497 at 503, [1967] 2 AC 224 at 237). That case was one of criminal conspiracy, but it seems to me that precisely similar principles must apply to a conspiracy for which a civil remedy is sought. Nor, in my opinion, can the fact that their ignorance of, or failure to appreciate, the unlawful nature of the transaction was due to the unfortunate fact that they were, as I think, erroneously advised excuse them (Cooper v Simmons, and see Shaw v Director of Public Prosecutions, where the appellant had taken professional legal advice).
— Buckley LJ. Belmont v Williams [1980] 1 ALL ER 393