I have a few questions and wonder if anyone can help me. I need some way to work out the pressure drop across a oriface plate or sluice valve at the point the valve(ball valve) is nearly closed, i.e where the first distinction between the background sound, and the sound induced by what I guess is the sonic velocity of the fluid (through the pressure drop through valve closure) is measured. Unfortunatley it is very difficult to measure the pressure the other side of the valve (outlet), so I need some kind of Physical law/equation (for water) that can derive this, that also considers the mains/pipe pressure (inlet pressure) as a variable. If anyone can provide any insight or maybe point me to where I can find the releavnt literature, it would be much appreciated.

In the case where you have choked flow, only the inlet P, inlet P, inlet molecular wt,gas ratio  of spec heats,  and the orifice area and geometry define the flowrate; the downstream pressure is irrelevant.

Very close to the outlet of the valve/ orifice, there will be a series of shock waves, and the pressure will discontinuously decrease across these . There are lab techniques to visualize these ( schlieren photography, laser anemometry), but I am not sure the results are repeatable nor can you determime the exact pressure drop across each distinct shock , but I could be wrong. See you local engineering school's professor of compressible flow.

Here is something that i had put together,see if this helps.This should be applicable to find the pressure drop accross the Gate/Sulice valve too.
The pressure loss (or pressure drop) in a pipe, tube or duct can be expressed with the D'Arcy-Weisbach equation.
?p = ? (l / dh) (? v2 / 2)
Where
?p = pressure loss (Pa, N/m2)