Cooling off period?

Tenancy Types and Management

We have AST agreements for whole house ,offering them to students. We offer a viewing, then if the students want the house we take an administration fee; then we reserve the house (ie. show no-one else) until the agreement is signed up to two weeks later. - The students chose the date. When the administration fee is collected and the house reserved we send out a copy of the uunsigned agreement for the students' inspection. Do we need a 'cooling off' period, either after the adminsitration fee is collected or after the agreement is signed. I know there is some legal requirement for eg. timeshare purchasing. Your advice is appreciated.