POWERMAX 155 - Will not heat water & radiators simultaneously ?

Hi all. I was hoping that someone could help? My Powermax 155 will not heat the water to the showers or taps, when the central heating is running. I have been told that it is fine and it's just the way the system runs, due to very cold water coming in, in the winter blah, blah blah!? but I recall having this problem in the past and a valve was changed!

I cannot see how any manufacturer would allow a system to not heat water and prioritise the central heating system instead. However I could and probably will be proved wrong here!

Also, 3 days ago my boiler started giving off a burning rubber smell? Transco came out and said that the system is safe, and not leaking gas. My insurance company sent an engineer today who said it is just the inhibitor in the water burning off, as I have had leaks from pump etc. (which are now fixed and pump replaced)

Any responses aree greatly appreciated..

Thanks in advanced