Ask an Property Solicitor. Get an Answer ASAP.
Under your lease, your landlord will have an obligation to repair, and maintain the exterior, structure and common parts of the building.
If your landlord has failed to repair the above you could make a claim for housing disrepair. Have you put your landlord on notice of the issues?
I understand why you are withholding the monies. It is important to note that this is a breach of the terms of your lease and your landlord could potentially sue you for the outstanding arrears.
I believe you have two options:-
1. Pay the outstanding arrears; or
2. Hold the monies in a separate bank account to show you are willing to pay once your landlord fulfils their obligations.
No problem at all, happy to assist you. Please do not hesitate to let me know if you have any further questions. Daniel