The socket outlet was part of a complete re-wire and was signed off by building control at the time.If the socket outlet has been installed and signed off by BC IMO your contractors advise is correct.
It's the same part P and NIECE registered installer who came back to look at the pond who said to just use the 13 amp socket outlet. He said it was already protected by the required RCD in the CU.
Before he came back I was ready to pay him to do any required work but he's a really genuine guy and said I would be wasting my money to do so.