Ferroli Optima 701 Hot water intermitent?

What is the cause of intermitent hot water. Central heating works without any problem but hot water is not reliable.We have called in Plumbing & Heating professionals who checked for system leaks because of pressure dropping found nothing.They then decided that sensers were the cause so changed 2 of them, this achieved a hot water supply for a few days and then went back to a state of hot water no hot water again.The heating engineer made a statement after completing the original repair that if this was not that answer to the problem that a valve would need to be replaced at an extra charge for the visit plus the cost of the part It seems as if they are only interested in making money. can anybody advise on the probable cause and if possible pinpoint the actual unit so that we can be sure that is the piece worked on.The appliance is the Optima 701.