Half the house has power - Stove, hotwater tank and dryer not working!
Your electrician seems to be spot on. The fact that only one unit is having problems leads me to believe City Light's connections are good and the breaker after their connections for the problem unit is at fault. Why didn't your electrician replace that breaker when he examined it? 
If this was a breaker in the unit I'd suggest swapping it with another breaker in the panel and see if the problem followed. Since it's a main unit breaker you wouldn't want to do that and have the problem move to another unit and tick those people off.