the amal valve gets its normal supply from the fuel pump relay.so once your pump is running the valve should be supplied, the valve is earthed through pin 16 on the ecu its the brown and yellow wire on the valve connector
you can break the supply or earth to the valve and you get the same result which is low boost because the valve is designed fail safe in other words if it either supply or earth wires lost continuity it prevents you overboosting you engine.
when ignition is switched initally you get a timed pulse to the valve for about 2 seconds to click on and then it clicks off.
you can give the valve any 12v supply but i think its best to earth it through pin 16 although i am not aware of any problems giving it its own earth as long as the supply is dead with the ignition off i think the valve is pulsed on the earth side to give gradual overboost protection instead of abrupt cut off when using pin 16,and you would loose this if useing a seperate earth