Why wont my Baxi Combi Boiler provide me with any hot water?

We have had 4 engineers out to our Baxi Combi Boiler over a period of 2 weeks. We have had no hot water at all and it seems that they are baffled. We have had the following parts changed as each time they come out they tell me its a different part that is needed and its getting extremely frustrating! Expansion vessel/heat exchanger/heat sensor and now a 3 way gas valve. Each time I go upstairs and try to run the bath/tap there is no hot water. Can someone please give me some advice as to what to do or what it could be that is causing the problem as I really need hot water!!!