First of all forget ideas about RTM as you must leave this in the control of the landlord*.
Second for the retained parts ( those not demised in a lease) it is always the responsibility of the landlord, always.
As owners you may have the defence of Defective Premises Act 1972, and possibly under Occupiers' Liability Acts 1957 and 1984, depending on the circumstances.
Those same defences to those costs exist for parts demised in the lease, as well as any contractual remedy between vendor and buyer.
As leaseholders there is the scope of the lease wording, and the LVT can take into consideration those defences in
determining a service charge.
Is there an NHBC or other building warranty such as Zurich's?
I suggest that you gather as much information on ownership and the leases and directly contact Counsel for advice, using a fund made up of those of you who can contribute or is one of you has legal expenses insurance.
Do not expect the LVT to deal with this well by personal application, I strongly suggest that you employ advisors as arguments in the above are complicated.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers