Harpic Power Plus Deep Cleaning Toilet Tablets, Mountain Pine, Pack 1 x 6 Clean Without Scrubbing, Removes 100% Limescale, Cleaner

Amazon

Cleans where the brush can't reach, no need to scrub Mountain Pine Fragrance Removes 100% of Limescale For best results, use 2 tabs and leave overnight Suitable for septic tanks

Zoro Tools

Harpic Power Plus Deep Clean Tabs Original cleans the toilet below the waterline and all the way into the u-bend, without the need to scrub.