The bond is money you pay at the start of the tenancy as a security in case you do not follow the terms of the tenancy agreement. The landlord/agent:
- cannot ask you for anything other than a bond in the form of money (you cannot be asked for a written guarantee, for example)
- can only ask for one bond for the one tenancy agreement.
The most bond you can be required to pay is an amount equal to 4 weeks rent (that is, the rent you agreed to pay at the start of the tenancy agreement).
The landlord/agent cannot require you to pay more (or another) bond because the rent is increased or because a new tenant moves in under the original tenancy agreement. You are not required to pay a bond before you sign a tenancy agreement.