When a leasehold flat is bought a standard question a solicitor must ask is 'are there any major works coming up?' If the company has issued a 'major works planned' letter to all occupants of the building that information should have been made available to a potential buyer. You must check if your solicitor asked that question and look for the letter in all the documentation your solicitor would have put together for you in your buyers pack. If the letter is missing then it's either the seller withholding information, your solicitor dropping a big ball or the housing company being useless? Do a bit of digging about and see what happened in that regard. If there is a major works letter in your buyers pack your solicitor should have advised you to negotiate down the price.

At any rate major works are part of shared building life. Any building will need work done over the years. Just try to figure out if there was a dropped ball in the purchase process.