UK Property Law
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Sadly your problem is not uncommon and you have almost answered your own question. A majority of you can forcibly buy the freehold under the 1992 Act. If the freeholder is unresponsive, you may steamroller the purchase through by making an application to the Leasehold Valuation Tribunal and follow its procedures until you reach the stage where you pay the purchase price into the Tribunal and the Tribunal makes an order vesting the property in the leaseholders. It is then yours.
If you don't want to go that far, the other way of getting the property put to rights is to form a "right to manage" company and take over the management of the building yourselves. You may then either organise the maintenance yourselves or choose your own managing agent, who will then act on your instructions.
The managing agents are just that, agents for the freeholder in carrying out the freeholder's responsibilities under the lease. I anticipate that your lease will contain the usual 'get out' clause saying that the freeholder must only maintain the building if the leaseholders pay the service charge. You cannot compel the managing agents to do anything. If you form a right to manage company, you may choose your own agents!
You ask the best way to proceed from a legal perspective. I am not permitted to give advice and am not doing so. You have the two options outlined above. Which one you choose will depend in part on finance (the right to manage route should be the cheaper option) and which you would prefer. The second option is the more final as, once completed, you can bid farewell to the freeholder and his agents!