******* boiler playing up. need help please?

Hi i have a glow worm 18sxi boiler. and yesterday i called out someone to have a look at it because i can ONLY get hot water when turning on the heating. or turning on the electric water tank. if i dont have any of them on, and just turn the boiler on and turn hot water on i wont get no hot water, and i told the man this, he aparantly checked it all out said it was fine, and the reason why i wasnt getting hot water was because my hot water settings were wrong, so he set them, and this monring i woke up and nothing! didnt do jack all and i have no hot water again!, any ideas people?