when fixing ones cistern valve, use nothing but Torbeck valves and parts.
Their valve stem to porcelain sealing washers have one spherical side to seal that huge 19mm to 25mm difference, the stem height, when set, does not infringe the cistern lid and the stem port is the correct shape and size to match normal 15mm inlet pipework.
I should have know better, a day wasted trying to fit a super duper piece of new tech sh 1 t.