Thanks for the replies guys. I cannot see the crack from inside. There is no water ingress. My first impression was that it's just, as you said, a surface crack and not one that goes deep. We had a rather rough soft-grounding last year and the boat was repeatedly lifted and dropped onto the keel before we could get out of it.
The yard has not dug into the crack yet, but they seem confident that it's a big deal. This sound odd to me, hence my post. They are a reputable yard. I'll ask them to do this before I hire a surveyor.