Well I noted the answer for shaving off a bit of the rubber inside the valve housing and as suspected the rubber had expanded and gone softer. Shaving approx 1mm off the expanded end did the trick. To test you can simply blow through one of the inlets and make sure it comes out of the associated outlet. Block the other two ports off with your fingers. Now the other side you will need to suck as its a shuttle valve and you need to draw it across its housing. You can do this test prior to dismantling it to ensure its not blocked by the rubber. Off course this is assuming that your pump is running and simply no water getting to either the front or rear washers. This saved the cost of a new pump and even a 2nd hand one would eventually suffer the same problem .
Pete Taylor, November 2013