I sorry, but I don't see how this is the boat yards fault
He was very clear in his instructions about what he wanted the yard to do. Grit balst and two pack the hull, re pack the stern gland, check the prop, rudder and anodes
No where did he ask for a hull survey. It's up to the owner to ascertain the hull condition.
The yard could well not have known that the hull had been compromised. A small hole at the bottom of a pit is not always easy to see and if the hull was already fairly pitted, they're not going to spend time going round every pit to check.