Letting agreement needed

Tenancy Types and Management

Hi
I am working away from home most of the time and am going to let out a spare bedroom to a couple. I need a letting agreement to rent this bedroom in my house, and share the use of the rest of the house.
I am not sure if I can use the standard RLA AST as the "lodgers" will pay a fixed amount that should also cover estimated additional services and council tax costs....unless I just cross through the line about the responsibility of these charges in the AST agreement and we all sign the amendment???
Is there another tenancy agreement that is more tailored to my situation?