Kettle Descaling For a Few Pence a Time!
Anhydrous Citric Acid. £12.99 for 2Kg. https://amzn.to/4dknJ7i
The last bag lasted me two years.
Rubber gloves and eye protection is required.
A spoonful or two into warm water and then wait a few minutes.
Use the water on a cloth to clean the outside.
Rinse a few times, boil and discard a few times and the job is done.