On Saturday, 9 January 2016 16:22:06 UTC, Roger Mills wrote:
The new fill valve is adjustable for height. It has a "critical level"
indicator and the blurb says that this must be at least 1" above the
overflow level. But if I do that, the top of the fill valve is too high
to fit the lid! What am I supposed to do?

I think there has to be a certain height (25 mm?) between the water inlet and the overflow level so that cistern water cannot back-siphon into the mains supply even in an overflow situation. If not, then use a double-check-valve in the supply.

I think BS 6700 and BS 12121 part 2 or 3 "Type B air gap" may apply, or is that for storage tanks not WC cisterns? Anyway the principle is the same.

Owain