More update:
The cause of the leak when on the slip trolley was easy to see once I was able to move Windchaser back further on the trolley. A very old crack underneath. The area in the bilge above this crack was where the soft black fibreglass rubbish was. And it was on the outside of the bilge on the wall above this point where the cracking happened when Windchaser came out of the water (see nxt post). I think that the seepage of water into maybe not fully wetted out fibreglass in the bilge over some years made it go black, like osmosis and weakened the whole area. That's why it flexed when on the trolley.
Ground the area back. Nothing on the other side where the crack was as the largest concentration of soft black fibreglass muck was right at this point on the inside.
The underneath section got several layers of fibreglass cloth with epoxy. I realised later that there was still a dent where the hole had been. Needs fairing but heck, I am still learning (and running out of slip time.)