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Under residential Leases, a Landlord is indeed normally responsible for any maintenance to the structure and common parts of the building. However, the Lease will also stipulate that each Tenant is responsible for reimbursing the Landlord their due proportion of any costs spent by the Landlord on carrying out repairs to the structure/communal areas.
What normally happens, therefore, is if there is any work to be carried out which is going to cost the Tenant more than £200, the Landlord is under a duty to serve notice on each Tenant of the proposed works he intends to carry out. The official name for this notice is a "Section 20 Notice".
You do need to check the wording of your Lease to see if the Landlord is entitled to claim back the monies he is going to spend, but I would be most surprised if it did not contain this provision, and if you have received a Section 20 Notice, it is pretty safe to say that this provision is included, and you will therefor ebe responsible for paying the £400.
I am sorry this is not the answer you were looking for, but it sets out the legal position.
Please let me know if you require any further clarification.