Hey Saffieboy,
You really need to mix epoxy resin with some microballoons filler and clean the insert up with some acetone prior to re-inserting it. Wash the area and flush it properly to remove any salt water, then let it dry completely. Clean away any foam/debris stuck to the insert and fill the hole with resin/filler mix, then insert the insert.
To make it stronger you should also roughen up the surface on the top deck and put a couple of layers of light glass fibre matting to give it some strength also as the top deck has been fractured and has lost all it's skin strength. If you are careful and keep it tidy, most of the work will be hidden by the footpad/footstrap.
Good luck,