Bio-D's concentrated Toilet Cleaner always comes up trumps; an independent test in 2015 found it removed twice as much limescale as the leading eco brand. It has a refreshing, lemongrass-based scent, and like all Bio-D products is hypoallergenic and free from petrochemicals, phosphates, preservatives, enzymes and synthetic perfumes. 

For best results, Bio-D recommend using the product under the bowl rim and leaving overnight, then using a toilet brush to remove dissolved scale.