You would need to prove that the infestation predated your occupation. In this case it seems likely.
If you simply decide that you’re going to move out because of this, you need to give the landlord ample warning to put it right. The landlord is then going to try to sue you for the balance of the contract.
What you might want to do is contact the Environmental Health Department at the local authority because if they say that the property is uninhabitable, that solves the problem for you.
Because this is a housing issue and regarding unsanitary conditions, depending on your income, you may be eligible for legal aid so it was the worthwhile seeing a solicitor that deals with housing disrepair on a legally aided basis.
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If you feel that it is not fit for living in now, speak to the property owner or the agent today and unless they agreed to look at this within the next day or so, then refer the matter to the Environmental Health Department at the local authority.
You can always use the statutory provisions to defend terminating the contract and it would be for the judge to decide whether the contract was sufficiently breached to allow you to simply walk away.
The general rule remember with property is that caveat emptor applies although there are certain regulations with regard to misrepresentation and things like “good condition” is relative and in the eyes of whoever is looking at it.
Based upon what you said, rather than have the environmental health Department on his back, the landlord would be better allowing you to walk away from this.