Apologies once more for resurrecting an old thread.
I replaced the OE nozzle with a twin jet one then about 12m later it stopped working again. It seemed clear, so at this point I took Daddycool's advice and checked the one way valve in front of the rear NS seat. Coincidence or not it (prized it apart and cleaned it as best I could) made a difference, so I replaced it with a straight one (rather than 90 degree) off a well known auction site. Everything seems good again.
I will add, though, that it is wise to test any cheap one way valves with a syringe and a piece of pipe before attempting to fit. I bought a couple to try and one was very restrictive, another did not work at all (was replaced no questions asked). If your pump had been working and the valve was stopping flow, once you disconnect the pipes to connect the new valve, washer fluid (under pressure) will go everywhere.