trace the pipe along and you will find a number (how many i do not know) of non-return valves. In these are little rubber seals that perish and allow the water to escape from them. Look under the bonnet and get a friend to press the button to wash your windows - you may see a stream of water escape from one. you may need a new valve, or if it is possible like I have done on a car in the past, you can prise them open and make your own repair.
There is every chance it may not be this - but I had the same problem and it worked for me so worth a look!
hope it helps
damnstudents, January 2005