Potterton Suprima 50?

I have a Potterton Suprima 50 boiler and the red light keeps staying on permanently (not flashing) and the green light goes on even though the timer is for hot water is on.

I have tested by putting the central heating on and the boiler then works normally. When I turn the thermostat up on the hot water cylinder the lights on the boiler again operate normally. Any suggestions?