Citric Acid is what I used as a professional small appliance repairer.
You will find it in the baking section of Woolworths or Coles, Very small container cheaper than a bag of lemons.
Fill kettle to its Max mark.
Bring to boil.
Disconnect from power and place kettle in Kitchen sink.
Use a teaspoon to a tablespoon in the very hot water and watch, the stain dissolves in a minute or two.
Tip the water out as soon as the stain goes i.e. about a minute or two, other wise if the kettle is copper,
with a chrome like lining, the the chrome will dissolve.
I clean my kitchen sink with it and toss in my stainless steel cutlery, experiment with it.
Google Citric Acid so you know what you are dealing with.
Citric Acid looks like sugar, so make sure it is clearly labelled in your Kitchen cupboard.
Put it well out of the way of children as it can be catastrophic if consumed.
Citric acid is used in cooking so is safe in small doses, BUT is not eye friendly so treat with caution.