This is a complex area of law, so if you need certainty, you should seek specialised professional advice.
My view is that the Parish Council owns the Parish Hall only as Trustee, and therefore has no ownership rights beyond those stipulated in the legal documents that created the present situation.
However, there is an obvious difficulty if the Charity administering the Hall becomes insolvent, something that is likely to happen if the Parish Council chooses not to pay for expensive maintenance. Exactly what would then happen depends on the precise terms of the documents and charity law.
It's also my view that these arrangements do cause a lot of trouble for conscientious councils, since the council has only very limited jurisdiction over how the charity is managed, but is expected to pick up the pieces when things go wrong. But I don't know of any solution to this problem. If anyone has a solution, I would be very glad to hear it!