Matthew.

The best way would be to take the deck off in that area and fix the stringers which may have broken away (assuming that there are strigers). If you use a router set at the deck ply thickness then that would give you easy access to see how big the problem is and get access for the repairs. Are you able to see through from the inspection hatches or feel if there is any damaged stringers ?

If there are no stringers and too much flex then you may have to add some stringers.

Naturally you will need a new section of ply for the deck after the repairs.

Regards,

Peter