Thanks all for your replies. My builder just thinks nobody will bother to inspect the work, or if they do it will be a cursory glance rather than demanding to see every inch of pipe. He bases that on his experience of building control, but I don't know if water company are more thorough or if...
I'm replacing lead and shared mains while my kitchen floor is dug up. Got on to Severn Trent and they've sent me a document with some rather unclear regs. There's some good info elsewhere on this site, I've also found this guide that gives slightly more info than ST...