What ball valve do I need for cold water tank?

B&Q have high pressure part 1 and part 2 and low pressure part 1 and part 2 valves. That's 4 valves to choose from. How do I know if I have high or low pressure and whether part 1 or 2? I rang my water Co and they said part 1 or part 2 doesn't matter and they don't "police" the regulations. I'm not convinced the person I spoke to knew what he was talking about.

Paul B
July 2010