Worcester 28 LE - lack of hot water?

The heating operates as usual but recently there's been a problem with the hot water supply. I've turned the thermostatic dial all the way up and we get scalding water at a low rate from the taps. The temperature of the water decreases a lot though when you open the tap all the way up. Also, the thermostatic mixer shower temperature is less than satisfying. Basically, when the flow is high, the temperature is low. What's wrong with this boiler?