UK Car Repair: Vauxhall Vectra, head gasket problem, water circulation

Question
My 1997 2.0l petrol Vectra is overheating.  Vauxhall replaced the water pump and thermostat but the problem persists.  They suggest it's a head gasket problem with a £500 charge to replace it.  The car's not losing water and the oil does not appear to be contaminated.  One of the coolant pipes gets hot, the other remains cold so I'm wondering if it's a problem with the heater matrix.  There's no heat coming through the heater.  Already tried flushing the system to no avail.  Can anyone help please?

Answer
If it is a head gasket they can test it and provide proof. BUT, why were the water pump and t/stat changed? From your description there is clearly no water circulation, so something is restricting the flow.  Has the radiator been given a simple flow check? It seems like the dealer did not approach this job sensibly.  Show them this letter and request that they check it properly.