water stays hot for a short time then runs cold

[mercer2773]

I have a Alpha 240 eco Combination Boiler, when I use the shower or hot water tap ,the water takes a time to heat up. once hot it stays hot for about 30 seconds, the runs cold. I hear the boiler burner start up and runs for a short while then cuts off. the pilot light stays on. when I use the central heating the rads warm up and stay hot, but the burner does the same .cuts it stays on for a short while and turns off again . I have bled the Central Heating System of any air and the system pressure stays within the green area on the pressure gauge. any ideas why the water does not stay hot regards

 

[Slugbabydotcom]

Diaphragm in the Diverter valve

 

[mercer2773]

Diaphragm in the Diverter valve

Thanks for the reply is this a fairly easy job to repair or do I need to call in a plumber