You must advise tenants of all material information and ensure that there are no misleading omissions from the information provided. This includes responses to questions from potential tenants.
When arranging for a potential tenant to view an already tenanted property, you must ensure that the existing tenant is given appropriate and reasonable notice (24 hours recommended) of the appointment and in accordance with any provisions within the tenancy agreement, unless other arrangements have been made with the agreement of the tenant.
When accompanying a potential tenant on a viewing, you should take appropriate steps to ensure the
personal safety of all involved.
The question of showing round prospective tenants in a property which is still tenanted can be problematic. It needs to be done sensitively as many tenants will consider it to be an intrustion on their privacy.
If there is no clause in the tenancy agreement then you will be dependent on the tenant’s good will and if the tenant fails to agree, you will have to wait until they have vacated before doing viewings.
In reality, even of there is a clause, if the tenants refuse to allow you access you will have to accept this. You will not be able to use your keys to take the prospective tenant round anyway. See 4.3.7 of the code.