Spoke to a company involved in the "supply chain" today. Respecting their
straightforward and honest answers no further details of identity.
Aside from telling me the taps were ok to 5bar the background to the
confusing instructions is an allegation there are "a lot" of legal actions
in progress between consumers and the "sheds" over water leaks from taps.
Some consumers have bought and fitted "bottom end of the market" taps with
no thought as to their water pressure and with no legal max pressure in the
UK there have been some substantial bursts/leaks. IIRC 15bar was quoted as
the water pressure of one complainant. So saying the cold feed should come
from the tank and the H & C pressures should balance absolves the supplier
of any "high pressure" leak whilst at the same time sneaking in the non
return valve requirement to ensure the legality of plumbing into the cold
main.

Thanks for the responses

Peter K