I went out to meet the Plumbers and inspect the damage (to the wallet that is) and were they surprised to see me. There was no rock. After a bit of back and forward we found that the plumber is doing a couple of jobs and he was talking to my SS about another job.
I don't care, it was worth the trip to get out of the office at lunchtime and to find out no rocks therefore no extra costs (YAY) plus another stage done - external plumbing. I think the next step is framework.