good point puternut ................. it's been standard practice for as long as I can remember to have mains-fed cold supplied direct to the bath and basin............. but, thinking about it......... there must be places out there that still have roof tank-fed cold taps in the bathroom.
Kassee .............. turn off the incoming mains............. probably under the sink ........... if the basin and bath are still running, then they must be roof tank-fed.......................... best not to drink it then ............