Tried that allready,doesn't work,just builds up the pressure then switches off.
No big deal i guess,just means i know the windows will be well rinsed!! 
Jarvy,
I have the wcw one too, have got on great with it for many months now...well...a year actually! Have full van mount too mind.
Regards turning the flow down.
Simply use a valve or inline tap. Turn it down too much of course and the pump will simply shut off, but it's a piece of cake to just adjust it to the flow level you want, and no, the pressure won't keep building up, nor will the pump keep cycling and thereby burn out the pressure switch.
You certainly don't need to waste money on an electronic flow controller unless you want to preserve and extend the battery life, eg, maybe your battery runs out of juice, ergo you need to conserve its output...not that you would fit such a flow controller to a back back anyway!!
Ditto on a full size van mount. My flow control if fitted on the negative pressure side of the pump so no matter what flow level I have the pump NEVER cycles.
You only need the potentially troublesome electronic flow controllers if you are worried about your battery running down during the day.
Ian