Thanks for your question.
You've got to go back to your original Lease and check the clause relating to forfeiture. Every Lease has one, if yours does not you would certainly have a claim against the solicitor who acted on your behalf during the execution of the Lease.
A forfeiture clause empowers the Landlord to forfeit the Lease (ie. to terminate it) so that the Tenant no longer has a right to occupy the premises. You can then sue the tenant for the outstanding rent under the Lease and would obviously be free to market the property for a new tenant.
Note that if the forfeiture clause does not contain the words 'whether formally demanded or not' then you will have to make a formal demand for rent before exercising your right to forfeit.
In the first instance you should write to the tenant a formal letter setting out the rent arrears. Entitle the letter "Formal Demand for Rent" and ask for payment within a specified time (7 or 14 days) and state that if the rent arrears are not discharged then you shall execute your right to forfeiture of the Lease for non-payment of rent citing the clause and the dated Lease. Explain that his right to occupying would cease and determine upon forfeiture and you would immediately sue him for the rent.
If you effect forfeiture I would strongly advise you to only effect peaceable re-entry outside of the tenant's business hours. The most practical way to do this is to turn up outside of these hours, check no-one is occupying and change the locks.
You can then sue him using the www.moneyclaim.gov.uk if the amount you are claiming for is less than £5, 000.00. If the amount is more then you should speak to your local county court and ask to be referred to their application forms of issuing a claim other than a small one.
You could theoretically exercise distress upon the tenant's property by taking it in order to force him to perform his rent obligations, but these is used only very rarely and you can run in to difficulties if you seize goods that are not his. I would recommend the above remedy.
You ability to re-let the property should also be considered. If you doubt it can be easily re-let then you may wish to leave the tenant in occupation clocking up rent arrears until you think you can re-let it. You can then sue for the increased arrears and then re-let.
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You would have to effect forfeiture by either apply to Court for an order for possession or peaceably re-entering the property to take possession of it.