You need specialist advice, as this is a complex area of law. There is some information on the website of the Intellectual Property Office.
The contract terms will usually say something about ownership of inventions at work, especially in engineering or other creative roles.
Otherwise, the general rule is that an invention made during the course of an employee's normal duties, or in the course of duties specifically assigned to the employee, is likely to belong to the employer.
There are separate laws relating to copyright covering drama, music and literature. Sometimes there is an express term in the contract specifying to whom the copyright belongs. In the absence of any such term, the presumption is that it belongs to the employer.