Water pressure on a Sime 100e combi-boiler?

... after replacing the diverter valve diaphragm, dhw heat exchanger and cleaning the inline filter I'm still left with one problem; low dhw pressure so that when any other tap or appliance is turned on at the same time a hot tap is on, the boiler stops heating the water. Is there any way of increasing the flow rate on the dhw circuit?? any help very much appreciated.