Damn....
After driving 40 miles to collect me diafram washer and then fitting it the inlet mechanism is still leaking very slightly and given the pressure of the water it's making a hissing noise inside the cistern. Whatever is causing the leak is very small as the water is coming out the form of a fine mist. I think there's more wrong with this than just a knackered rubber washer.
I might have to call a plumbder because if I start messing any more I'm bound to make it worse.
Did you accidentally drive round your little island three times to do 40 miles?