
Ok, I consulted the oracle that is.... My Dad
If the stopcock is plastic above and plastic below he doesn't see why that wants earthing in the first place, plastic don't conduct lectrickery.
He said some rude words about earthing your sink to a stopcock many rooms away, and said to tell you to ask your electrician/plumber/builder whoever it is.. to SHOW you exactly where it says that is necessary in the latest edition of the IEE Wiring Regulations (the latest edition btw is the 17th)
He suspects the guy is working from an older edition and what is suggested is not necessary in a domestic setting with presumably mainly plastic pipework and modern electrics.