hi i have a Worcester greenstar 24i junior boiler ,cental heatiung working ok and the pressure on boiler is fine ,when I run hot water for a bath the water comes through hot for about 5 to 10 mins then shuts the boiler down and the reset and blue buttons start to flash,only does this when I run hot water,any ideas what is best to change first,also is it best to call Worcester bosch engineer to fix problem.many thanks.