The tenant is bound by the length of the contract. You can serve a notice for possession on the tenant when the tenant is two months in arrears (if the rent is paid monthly) with his rent under Ground 8 s8 Housing Act 1988. The notice period for this is two weeks and the notice must be in the correct form (you can download it from the online document providers).
Once the tenant has vacated (presumably he will not actually be in occupation) then you can sue him for the rent under the contract using www.moneyclaim.gov.uk. You would not get judgement for the whole of the 6 months rent because the Court would take the view that you would having mitigated your loss by reletting to a tenant. You would get judegment for the rent that you would have received up to the time at which a replacement could be found (2/3 months most likely).
In the circumstances it would seem appropriate (quicker for you to re-let and still get some money from the tenant) to suggest a surrender of the tenancy in consideration for the tenant paying you a sum of money so that you do not sue him for the rent. He is bound by the contract (and the rent) unless you release him by surrendering the tenancy so suggest a figure to him and see if he agrees.
If he does then you can prepare a document referencing each party, the tenancy, the dates, the property, the payment of the sum you have agreed and declare that this money is paid to surrender the tenancy so that each other's rights then cease. You should both execute two copies of the agreement and each keep a copy.
Once the tenancy has been surrendered then you can re-let the property. It may be a better practical way of getting money from the tenant - if he disappears then you may have difficulty enforcing the judegment for the rent you would receive at Court.
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