hot water?

Linea 24 combi,central heating up ok,water is warm then cold when i run the bath and boiler cuts out water is still cold when the tap is just opened,warning light is green as if all is okay an engineer says the boiler is obsolete and needs replacing its as if the thermostat thinks the water is up to temperature,just wondered if cleaning the heat exchangerwpuld help?or is there anything i can try thats reasonably cheap/i should add i am housesitting for a friend who is in Australia for 6 months could it b a pcb? gas valve?any help much appreciated