I employed a building firm to build a loft extension on my 3…

I employed a building firm to build a loft extension on my 3 bedroom house.The relationship started off well, however as time went on, the relationship soured due to the incompetence of the staff employed by the building firm. In the end, I chose to employ a third party to rectify the insufficient and unsafe work carried by the firm. This third party went on to complete the actual job.Fortunately enough, I had not paid the building firm all the money they had asked for; meaning I was able to cover the cost of the majority of the remedial work using the money which would have been paid to the firm. The building firm acknowledge their failures and are happy to cover the cost for most (but not all) of the remedial work and have asked me to pay them the balance.The main reason we selected the building firm in question is because we were led to believe they DID NOT use subcontractors. On their website and in the agreement we signed it is clearly states that no subcontractors will ever be used. We later found out that 95% of the people who worked on my loft were employed on a subcontractor basis. I have recorded conversations and text messages which prove this. As such, I believe this to be a misrepresentation of the truth and a breach of our contractual agreement. Therefore, I believe I am no longer obligated to pay the remainder of the balance. I presented this to the firm and asked them to confirm or deny my allegation that almost every member of staff that worked on my property did so on a subcontractor basis. At first, the firm tried to lie and pretend that my assumptions were incorrect. However, when I asked them to more specific, the firm told me that their lawyer informed them that they are not obliged to confirm or deny whether or not subcontractors were used.So, I would like to understand 3 things:
1) What is my legal position? Do I have to pay them?
2) Is the use of sub-contractors a misrepresentation of the truth and therefore sufficient enough to void the agreement?
3) I asked them to specifically comment on whether or not named individuals that worked on my loft did so under the "sub-contractor" banner. According to them, their solicitor believes this is information they are not obliged to give me; even-though our agreements categorically stipulates that NO subcontractors will be used. Is this correct, or are they obliged to tell me?