Thanks for your question.
I'm afraid you will be bound by the full term of the tenancy agreement unless there is a break clause in it entitling you terminate at a certain point during the term or the property is unfit for human habitation (which would also entitle you to terminate on the basis that this is a repudiatory breach of contract which goes to the heart of the contract so as to deny you the substantial whole of the benefit of the contract..
The landlord is not responsible for the conduct/cleanliness of the tenants in their building, indeed it is his choice whether to enforce any breaches against them. If you do not like the tenants you have taken a tenancy with then this is really your decision unfortunately.
You should write to the landlord asking for the light to be fixed within 7 days otherwise you will do it yourself and seek to claim the money from him via http://www.moneyclaim.co.uk/
If you simply leave then the landlord may sue you for the outstanding rent owing under the tenancy agreement. He would get judgement on the amount of rent he would have earned up to the point at which the court decides he should have been able to re-let the property.
However, landlords are generally under a duty not to unreasonably refuse a suitable tenant and your focus now should be on finding one. If the landlord acted through agents then you should explain your situation to them and press them (and continue to press them) to find you a replacement tenant. If the landlord acted on his own then you should speak to him to check that he would be amenable to your finding a replacement tenant. You can then use http://www.gumtree.co.uk/ to find a tenant (private landlords frequently use this service.
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