We have an employee who has handed in his notice, giving 3 months notice, his contract states he only has to give 4 weeks. For business reasons we want him out earlier, as he is looking to start up a business in competition. His contract states that he only has to give 4 weeks notice, can we respond to his resignation letter stating that we do not require him to work 3 months notice, and that he will finish in 4 weeks time?