Sometime last week I noticed that the hot water in the house was all of a sudden REALLY hot

We haven't changed anything but I mean its nigh on boiling, if you get in the shower you have to put the tap to mainly cold to even be able to stand under it ?

I've checked the timer at that is actually fooked as we can only have the system on or off so as it appears to be working its arse off 24/7 I imagine the next bill is going to be high

What changes the temperature of the water ? I presume the central heating and tap water is seperate ?

Please excuse my lack of knowledge on this subject, I'm currently ill and not actually complaing about the hot water but I know its not right so if I can fix it I will or someone will need to take a look at it before it packs up all together