How to fix my worceister boiler? Junior 28i combi?

Hi there
I have just over 2 years old combi worcester 28i boiler and ever since we had to drain the system we have been experiencing problems with hot water. It is sometimes runninig and sometimes not. The pressure is correct and the central heating is working fine. We had it serviced few days ago and an engineer couldn't find a fault as the water was running normally. There are times where it is working fine and others where we can't get it to fire up. The boiler has been serviced and the guy said that it could be an air lock but he didn't do anything about it. We tried to empty it again and than refill it and put the heating on and let air out of the radiators but it didn't fix the problem. Is there anything we can do to check what is wrong? (how to tell whether it is flow turbine or divertor ass?) The fault light is not showing. Do u know if it would be a costly repair? Maybe it's worth mentioning that we live in an area (Reading) where we have very hard water, could that have sth to do with it?
Many thanks.