Thanks for yoru question.
THe basic position is that you can terminate the contract if the property is unfit for human habitation, this would mean that the landlord has breached a fundemental term of your agreement in such a way that the breach goes to the root (ie. the heart) of the contract to deny you the whole (or substantially the whole) of the benefit of it (called a repudiatory breach).
You've done the right hitng in calling enviro health, keep pressing them to come round and take a look at it. If they declare it unfit for human habitation then you should get a statement to that effect from them and you can use this as evidence of his repudiatory breach so that he will not be ablet to sue you afterwards. They may serve an enforcemnet notice upon him requireing remedial works to be carried out.
You could claim it as repudiatory now and terminate but he is more likely to litigate if you do not have a statement from the enviro health.
At the same time I would write to the the landlord/agent specifying the disrepair, making a list of the reasonable repair required and ask that he make those repairs within a reasonable time (eg. 7 days). State that if the landlord does not make the repair within that time you will pay to have the repairs made and will seek to claim the expense from him and are prepared to make an application to court if necessary.
If the landlord does not make the repair you can pay to have it done and then write formally to him requesting the payment of the cost, again within a reasonable time). If he does not pay you can issue a claim for the money yourself through Her Majesty Courts Service's online service: http://www.moneyclaim.gov.uk/
Its pretty cheap and straightforward to use. Needless to say you should document the disrepair by taking photos of it now.Keep copies of everything.
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