Yes. You, or someone over the age of 18, needs to be at home for the length of the installation.
It's important to remember that the appointment slot marks out the times between which the engineer will arrive. For example, if your slot is 10am to 12pm, the engineer could arrive at 11.45am. They’ll call in advance to give you a rough arrival time.
Once the engineer has arrived, it can take up to 3 hours to have a gas and electricity meter fitted, or around one hour for electricity only, so please allow yourself enough time to be home for the whole appointment.