Much of the UK experiences hard water. This map shows the worst areas. The most common sign that you have limescale is a white residue in your kettle. Although white vinegar can also be used to remove limescale, citric acid is slightly more effective at tackling scale build up. In both cases, a warm solution speeds up the reaction process. If the limescale is visible, you will see it actually fizzing/dissolving in kettles (but obviously not in the washing machine).
Limescale build up in your washing machine means that you are wasting electricity and it can ultimately reduce the service life of your machine. Remove any existing limescale by running a maintenance wash using a whole 250g packet of Citric Acid. Repeat the process at least every other month and help reduce limescale build up in the first place by softening your water by adding soda crystals or borax substitute in your regular wash. You can also use dissolved citric acid or white vinegar in the place of laundry softener.
Citric acid can also be dissolved in water and then used to descale your iron, although you can also use white vinegar neat.
Here's our blog post on washing machine maintenance.