Hello I am solicitor with 20 years experience I will try to answer this.
1) if you walk away the starting point is that you will be liable for the rent for the whole of the unexpired term. However, the landlord has a duty to mitigate his loss so if he doesn't market the property or turns down suitable tenants then he may find it hard to claim the whole rent.
2) If the damp is caused as a result of disrepair to the structure or exterior of the property then the landlord is in breach of his repairing obligations. If it caused by condensation then (unless this is caused by structural problems eg penetrating damp putting too much water into the house) the landlord will not be in breach of his obligations unless there is an express clause to the contrary in the agreement (unlikely). Your remedy for this would be compensation (probably calculated as proportion of the rent eg if 1 room in a 3 room flat was unusable you would claim 1/3 of the rent for the period of time the landlord knew of the defect and did nothing). You could raise this now and demand repairs and threaten to bring in the environmental health dept of the local authority.
Yes that is as I thought - it is a possible solution but does not give me closure straight away which I would like. I have just bought a house and cannot risk getting a demand for rent payments alongside my mortgage payments.
This would not mean your landlord would get a right to possession before the end of the tenancy but might make him agreeable to let you leave early. This will only apply of there is structural damp so you need evidence of this.
You can also use the disrepair as a counterclaim to set off against alleged arrears in any action for arrears brought by your landlord.
3) If you have incurred costs carrying out repairs which were necessary and the agents on the landlords behalf did not carry out then you may have a legitimate argument to set that off against rent owed as well. The merits of any such argument will depend on the details.
It may well be that you are in a position where if you negotiate well your landlord will think that the risk of retaliatory moves by you mean he should simply let you go.