Heatline S30 does not continue yielding hot water in Bathroom ?

My boiler located in the kitchen seems to be working fine for central heating and hot water in the kitchen is also working fine but bathroom hot tap and wash basin hot tap release only cold water. When you turn the bathroom hot tap the boiler kicks in but stops after about 30 seconds and the initial luke warm water turns into cold with continuous flow of cold water. Some gas engineers said heat exchanger needs changing but I would like an expert opinion on this, please, if anyone has.