Thanks for all your replies, i've had volkwagens for around 16/17 years now and i've had leaks in most of my cars and usually find them. i know its a process of elimination. I've replaced two pipes and the radiator and thought i'd fixed it. I did notice two pipes are joined by a solid plastic section, so this may be the problem? This pipe is the one going into the heater valve. is this what you were referring to ?