Sounds like a similar problem to mine - Diverter Valve. I don't know how much fixing this problem should be but the quotes I got were £300 and £320 and i thought them a little steep!!
The problem with little or no hot water has steadily been getting worse and now I have none at all and when the hot water top is switched on the system makes a terrible noise. I have heat getting to my radiators just fine but not to the taps. I've been told it may be a problem with the diverter...