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. If they do not pay next months rent then you can serve this notice upon them.
If they have caused damage to the property then you can serve a s8 Ground 13 notice upon them citing that the condition of the property has deteriorated as a result of their actions. The notice period for this is two weeks.
It is very important that you use the correct form of notice. If you do not then a court may not make an order for possession. You can google the above notices and "document providers" and a number of online companies can provide them. Really though, your letting agents should be able to advise you of this and do this for you.
Once the notice period has expired you will have to then make an application to Court for an order for possession. You cannot just change the locks unfortunately, that would be an illegal eviction and the tenants could sue you.
You can also make a claim for a money judegment for some of the remaining rent at the same time. Your local county court will be able to provide you with the forms.
If the tenants do not leave after the order for possession has been made then you will have to apply to Court for a warrant for execution so that the court bailiffs effect eviction.
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