Vaillant Turbomax 242E problem?

Would be very grateful for your help here, I have a Vaillant Turbomax 242E combi boiler that in the last Two days will only deliver hot water intermittently, the central heating appears to be working normally, could this be the rubber diaphragm (Is this within the diverter valve?) becoming weakened ?
I have had a similar thing happen a couple of years ago which the Vaillant engineer sorted, however having looked at the split diaphragm and associated little bits that he left behind I could'nt help but think that there could'nt be much to fitting one of these diaphragm's, (I'm pretty handy mechanically/with spanners) am I correct or is this more difficult that I am assuming and what exactly do I need to know/does the job entail ? (Draining boiler/isolating feeds etc) your help/advice would be much appreciated.