I have an overflow problem which has not been resolved by differnt plumbers?

Ball valve has been changed and the iron bend but still overflow problem continues.Last engineer suggested to have a look at cylinder thermostat/boiler thermostat as the o/flow water was initially warm and then becomes cold.When water is used in the bathroom the overflow stops.My boiler is Vokera.The hot water cylinder and cold water tank are in the same place in the loft.Please help as this is driving me crazy