I need to find a valve, or some technique to heavily constrict the
flow of water in a 4cm pipe. Pressure is low coming from a 2m cistern
head above.

I have been searching in vain for a very cheap PVC butterfly valve
with poor closed sealing characteristics, but with no luck. I did have
a little more success with gate valves like this one
(http://www.valterra.com/Ind/Gate.htm) but as I need to actuate the
valve from a small motorised actuator I am concerned about the force
required to open/close the gate.

I am open to any valve technique provided it costs well under ten
pounds in quantity.

Does anyone have any suggestions as to where I might find a very
inexpensive plastic valve for this project?