I would ensure the max and min burner pressures are correect before deciding that you need a new heat exchanger. So many things it could be really. If you are not Gas safe I'm sure your engineer will be able to check.
Hes wrong its the heat exchanger.
Turn your nearest tap on just enough to fire the boiler and if it stays hot theres your fault.
Youre Hot water Heat Exchanger has absoloutly nothing to do with your central heating side or you would have been drinking inhibitor since the day it was fitted.
GET A PRO!!!!