Do you know if your Solicitor obtained standard replies to Leasehold enquiries from the Managing Agents? (One of the questions asks-"is the Freeholder/Managing Agents aware of any major planned expenditure in the foreseeable future?").
Also, having spoken with the Agents, have you been able to find out if the Seller was aware of these major works, and if so, I take it that this was not disclosed to you at the time of your purchase?
I look forward to hearing from you.