(3)In proceedings against any person for the infringement of a patent, it shall be a defence to prove that at the time of the infringement there was in force a contract relating to the patent and containing a condition declared unlawful by this section:Provided that this sub-section shall not apply if the plaintiff is not a party to the contract and proves to the satisfaction of the court that the restrictive condition was inserted in the contract without his knowledge and consent, express or implied.
