How can my Worcester condensing combi boiler be fixed?

We have a Worcester Greenstar condensing combi boiler and after 6 trouble free years, water keeps going cold. first noticed in shower and hot for 2 mins then cold. have to turn tap off and on again half way and keep doing that to get hot water for about 2 mins at a time. Similar probelm when filling the bath. Had diverter replaced but problem back after a week or so. no problem with heating - help!