I would suggest initially having a read of the insurance policy document. I know that they are very wordy but I am uncomfortable leaving it to insurers alone to make a ruling.
Whale, that is interesting what you say about a likley exclusion of the gradual occurrence. I do not have experience in the area and cannot comment apart from I can see some logic behind it as it’s purpose could be to exclude claims that are a result of a lack of maintenance at the appropriate time. In this case however, sure, it has been a gradual leak but if it was not detectable (due to the false ceiling), you might have a case to argue with the insurers
A false ceiling would be regarded an acceptable structure to build within your Lot, although the insurer might argue that an inspection panel should have been provided to check for the very problem that has occurred.
Is it possible that the false ceiling was installed for the purpose of concealing the staining caused by the leak? If so, then you may choose not to reinstate the false ceiling, in which case all work will be OC responsibility.
At the risk of stating the obvious, could I stress to you that the most important issue for you, in my view, is not who pays for the false ceiling, it is will the cause of the water leak be permanently fixed?
Regards,
Rob T