How can i fix my flow control valve on an Ariston Eurocombi A23 ?

Hi, please could someone help? My flow pipe has a little turning control valve (screwdriver goes into the little slot) that is leaking heavily. I had no hot water for 3 days. I got someone locally to come look and he said there was nothing wrong with the water after turning the taps on FULL (sending it all over the sides of the sink as it was so fast), but got hot water. I tried to turn the flow down after he left but it kept turning and is now leaking really badly and i still have to have my tap on full to get hot water. Please how can i fix this. Many thanks for any answers submited.
Selena Born x