Potterton Flamingo 50?

Hi
I have just bought a house having Potterton Flamingo 50 as a boiler. I am not quite aware of how boiler works but it has got a a water tank and a boiler on top with a water temprature control. The switch of water tank is turned off but the burner unit is kept on to keep the water hot.

The problem is whenever I turn on the heating the normal hot water in taps and shower becomes extremely hot as well. I have tried to decrease the water temprature control on burner unit to minimum but then heating becomes slow as well. How can I control the temprature of hot water separately from the heating system? - thanks