How many months in arrears is the tenant?
Are you not the owner? Are you asking on the owner's behalf?
If the tenant is over two months in arrears and the rent is paid monthly then the landlord can serve the Tenant a s8 Ground 8 Housing Act 1988 notice giving the tenant two weeks notice to vacate on this basis.
A s21 can be served at any time during a statutory periodic tenancy but the landlord must give the tenant 2 months notice with such notice to expire at the end of a rent period.
You can download a template for the above from one of the online document providers. Allow three working days for postage and send by registered post.
Once the notice period has expired and if the tenant has not vacated then the landlord must apply to Court for an order for possession. If there is a written tenancy agreement then the accelerated possession procedure can be used so that there is no need for a court hearing. If the landlord wishes to get a money judegment against the tenant at the same time as the order for possession then they cannot use the accelerated procedure.
If the tenant does not vacate following the order then you will have to apply to the Court for a warrant for execution so that the Court bailiffs effect eviction.
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