I have started work on the cracked deck in the starboard side bench in the cockpit. The multi-tool cut out the triangular area perfectly. Very easy to plunge into the plywood and control the straight cut. The crack was bad, and surprising. The 6mm plywood looks quite robust, so why it cracked in this area I don't know. Not a highly stressed place, although I do stand on it when going onto the stern deck.
I have cut out two 6mm marine ply patches and coated them with epoxy. The larger goes under the hole and is epoxied top the underside of the deck. Held in place with string loops tightened with wedges.
When the glue is set, the second patch, which fits the hole, will be epoxied on top of the lower layer. This should be stronger than the original deck, if I make it properly. I will probably put a screw in at each corner, just for back up, as I don't trust my gluing all that much. Once all set there will be quite a bit of tidying up to do. (I am resealing all the hatches in the vain hope of making them fully watertight).