Is there a pool of water under the car? No, I'm not taking the p*ss, same happened on my twinspark, turns out the non-return valve had literally burst itself, found the pieces under the plastic grill beneath where the windscreen wipers attach.
Got a new one from Halfords for a couple of quid.
You could also check under there for a trapped tube.
Will check out whats under the plastic grill this weekend,thats once i've dug the car out from all the mud and dust thats collected on it while the water mains have been replaced in my street! eek! eek!
I have removed the NRV from my car altogether with no problems and my 75 was the same. The NRV appears to choke quite easily also there is a filter on the pump inlet which may have choked. you can access it by pulling the small pump out (the large one is the headlamp washer) it is just a push fit. If the filter does not come out with the pump you can pull it out also. it looks like a locating grommet when it is still in position. Good Luck!