1kg Citric Acid Kettle Descaler 2 years of descaling Anhydrous Food Grade - Home Bath Bombs, etc £5.19 @ eBay
This is the active ingredient in expensive and tainted products like Oust that cost a fortune.
Couple of spoonfuls of this in your kettle, boil it - watch the limescale disappear - empty and rinse - job done!
Great for shower heads too. Don't waste money on branded products. Obviously there are other sellers shifting smaller quantities if you want to try.
Any links for cheaper would be helpful as would comments on it's effectiveness for the uninitiated!