Kingfisher Potterton Boiler?

Hi,

I have a kingfisher Potterton floor standing boiler (old system with water tank) and for the past 2 days the heating is working fine but the hot water does not come on . I have checked the timer switch which is fitted on the boiler itself and both the hot water and central heating in on however the light on the hot water keeps blinking and makes a ticking noise .

Before I call someone in just wanted to know if anyone have any idea what it could be as dont want to get done over .

Thanks in advance for all the answers