How do I fix my Ariston Microgenus 24 HE MFFI?

We've just moved into a new apartment which has a Ariston Microgenus 24 HE MFFI Boiler.

The boiler works fine, in the sense of heating water, however there are 2 issues with it.

1) the timer dial which dictates when the boiler goes on and off does not work (Not a huge issue)
2) There is no way to segregate Hot Water from Central Heating - when we switch the boiler on, its heats water and once it reaches around 45 Degrees it switches the central heating on as well - this is a pain as it means we have to constantly turn the boiler on and off every time we want a little hot water, otherwise we end up with a furnance.

Does the 2nd issue sound like a broken Diverter valve, and if so, how easy is it to replace this valve.

Any help greatly appreciated!