Combi-boiler - lack of hot water

[red3000]

I have a Sime combi-boiler installed in my new flat, I'm told it is only a few years old.
The pressure etc is all fine as a gasman recently came to replace my pressure valve and replace a valve on one of my radiators that wasn't heating up as much as the rest.
Since living there the hot tap on the shower has always had to be turned on full, but with the weather getting colder the water temp isn't as boiling as it should be. When I shower the hot tap is up full and I don't use the cold tap at all. What is the problem?
I think my bedroom radiator (closest to the boiler) needs to be bled because it taps loudly and doesn't heat as well as the others. Where does the hot water for my shower come from and why is it not heating up as much? Is it affected by the radiators or are the two not connected at all?
Any advice would be appreciated.

[gasman]

They are not connected the hot water is seperate from the heating side. Try turning the cold water inlet valve to the boiler down a touch ( aprox 11ltr per min should give you a 40 degree rise in temp ). So the cold water has more time to heat up as it passes through the heat exchanger.

[red3000]

Thank you. Where would I find the cold water inlet valve?

[gasman]

Just underneath the boiler there will be a series of valves two large 3/4" ones that will be for the flow and return of the heating circuit, One that will probably be yellow and thats the Gas valve and one smaller 1/2" one that is the cold water vale you need to rotate a bit.