Under no circumstances should anything other than citric acid or vinegar be used as other substances may cause irreparable damage. You can either:
Mix one sachet of citric acid (10 g) with 200ml/7oz of water. Pour the solution into warmer. Heat for 10 minutes on setting 3. After 10 minutes, switch off by unplugging from the wall and leave to stand for 30 minutes. Pour the solution away and rinse the warmer with tap water. Citric acid is available from most chemists.
Pour 50ml/2oz of vinegar (white) mixed with 100ml/4oz of cold water into the warmer ensuring the basket is in place. Allow to stand in the unit until any limescale has dissolved. Empty the unit and rinse thoroughly.