What you need is a "Work for Hire" agreement that states all creative works as a result of the employment belong to the employer (you).
There is a free template here:
free-legal-document.com/ work-for-hire-contract.html
(remove space from URL)
This contract is written for a US market, but our copyright laws are the same as the US, but our copyright laws are largely the same since UK and US are both signatories to the Berne convention. (I used to do this shit for a living, LOL)