We averaged 6.9kWh a day last autumn (Sep/Oct/Nov), and 7.8kWh a day during the winter (Dec/Jan/Feb). Two adults and two pre-school age children, all home all day. We use the tumble drier couple of times a week at least, wash maybe five loads. You're still getting on for double what we use.
First, I presume that whilst your gas combi boiler might run on gas, the pump which actually moves hot water around the heating system is electric? That's about 60W when the boiler is running for a start. If your heating runs a lot this will add up.
Our fan oven is 900W, so 90% of the consumption of a one bar electric fire when it's on. If you're running one for, say, an hour most days, that will add up enormously.
Our vacuum cleaner uses up to 2200W, which for the five hours a day my wife thinks it should be running is a phenomenal use of electricity.
Chest freezers are notoriously inefficient. My last Whirlpool one had an efficiency rating of E and then the thermostat broke so it ran continuously! Check this.
You mentioned two teenagers. Gaming devices (for the cooling fans) and hairdryers are the two obvious hidden drains that spring to mind.