You cant expect him to spot everything first time.
Once its brought to his attention you should ,if a reasonable character offer to pay more for the extra work involved.
I might be way out here.
But if the shower mixer valve has gone . When you open a cold tap it draws the hot water through the shower valve. This in turn fires up the boiler. It might just be a fluke that its started since you have had some work carried out . It might be worth checking out
If its a gate valve quite often these things shear off and drop into the valve. Either change it or just change the head. The biggest proplem is draining the water out or the water tank .if the gate valve is blocked