i'd certainly be pushing for some money off for mis-diagnosis and fitting parts which hasn't cure the problem
EDIT - just spoken to my missis who works for Trading Standards - she says to refuse to pay for the labour of fitting the water pump and the water pump itself
did you agree to the replacement of the cambelt?? otherwise you could push for not paying that.
She also said to ask them to PUT BACK the old parts as they aren't faulty and at their expense!!