The gradual exclusion relates to water seeping from the defective/deteriorated grouting to the tiles. In the event that water escapes from a shower enclosure due to deterioration of the grouting, the policy exclusion will apply.
If there is a leak from the waste of the shower or a pipe feed to the shower, this should not be excluded. However, the OP will have to show insurers and prove that it is a leak from the pipe which has given rise to the damage.
What complicates this case somewhat is the recent inception. If on investigation, it is determined due to the extent of damage to timber joists etc that the leak predates the inception of the policy, then Insurers may likely advise that they will not deal with the case or alternatively, only contribute a proportion of the cost of the repairs.
Under the Customer Protection Code, the OP can appoint their own regulated Loss Assessor to deal with this claim on their behalf.