how can i increase the pressure on my dhw from combi boiler?

Hi I live in a masonette on the upper floor. The council made us change our boiler to a combi one. But since we have done this the flow rate to the taps in the bathroom above is terrible. you can only run the shower on either hot or cold but can't mix. when mixed it becomes very cold quickly. is there anyway i can boost the flow?
Please help