If you get a gas tank, be careful who you go with as I believe only the supplier can refill the tank, you can’t shop around.
Oil tanks on the other hand, anyone can fill it. Unfortunately, anyone can empty it as well. Gas tanks are theft proof.
Your cylinders, on the neck of each bottle should be a tare weight…the weight of the empty bottle. Grab your bathroom scales and weigh them, if its considerably heavier, there is still gas in there 🙂
You can also get magnetic strips which detect the level of the gas, downside is they only work when you have been drawing gas for a while as its the liquid gas boiling off into vapour that creates a cooling effect, and allows the gauge to work. Not sure if they are used for domestic situations but its common to use them on gas bottles for campervans/bbq’s. If you are drawing lots of gas you will see the condensation line on the bottle, this is the gas level.
Also, check what the flashing boiler light is for, it could be for various other faults such as low water pressure or a boiler fault.