How can I fix my Glow-worm 50ff boiler?

We have moved into a new house recently which contains a 3 year old Glow-worm micron 50ff bioler since we have moved in we have had to have the pcb replaced. It started acting up the very next day after we had the pcb fitted so we called the engineer back only for it to work ok while he was there. Since then the hot water has gone off but the heating worked so we called out the heating engineer yet again (yes you guessed) it worked fine while he was there now the heatings off and the hot water works can anyone please advise.