Boiler has heating but no hot water for Alph InTec 26C.

[Stuart20580]

I have an Alph InTec 26C. I have never had a problem with it until recently. All of a sudden the hot water does not work. The boiler fires up immmediatelt to full burn when you run the tap but after 2-3 seconds it cuts out. It then tries to fire up again and again if I keep the tap running.
I have checked the heating side and this is fine. It goes through its sequence of low burn and gradually increasing until it hits the desired temperature. I checked the error code and it says 38. Before having parts changed I thought it could be the diverted valve, but this works fine as when I have the heating running and then turn on the hot tap and press it in, hot water comes out. However when I run the hot tap when the heating is on, it cuts out the heating.
So I have now had a new fan, ignition electrode and flame sensing electrode fitted. This is because the fault code said it could be these. However I still have the same problem.
Can anyone help?