the cheap monitors are not very accurate at low usages.
they tend to grossly overindicate usage by small items such as phone chargers, and make people think that their cost is more than negligible.
It is 427 watts per hour consistently.
If you mean the usage is 427W, which is 0.427kWh per hour, then the cost would be about 5p per hour. This would be about 20 energy-saving lamps, or a very small electric heater, so if it is a single appliance the heat should be very noticeable.
The total amount of electricity used was 7.7kwh over 180 hours.
However this is 42.7 Watts so you are still saying the wrong numbers. If you had a fridge or freezer plugged in, or any lighting, you should deduct its usage
Does this correspond with what your electricity meter says? Your meter may have a flashing LED which counts fractional usage.
You can turn off the MCBs one at a time if you like to see which circuits have your "baseline load". If you little device still claims there is a usage when all MCBs are off, it is probably wrong. Turn off the main switch and see what it says. Observe the electricity meter after an hour.
Your garden lighting certainly SHOULD have an isolator or an FCU in the house enabling you to cut all power to it. Try that. If it hasn't, get one fitted. The garden circuit should be protected with an RCD.