you are using the rent-a-room scheme and this is a licence agreement. It helps to have something in writing espcially a note of start date, what to do in the event of leaving (month's notice), the amount of the rent, how you will accept payment, what you will do about bills, and your arrangements for domestic duties(they can be a source of tension - if its your place the other person can expect you to do all the work). If you want to build the relationship as after all you will be living with this person I'd suggest that you sit down and write it together on a blank piece of paper and both sign it, keep it simple and to the point, no legal language, just two pals setting this out so that there is clarity for both. Then you make a copy and give the other person a copy.