My dm recently visited and brought this stuff with her to see what I thought of it. AMAZING! Since we moved in here almost 8 years ago, there's been a grotty line of limescale round the front rim, have tried everything over the years. Tried this and next time I went for a pee an hour later, gave it a wee rub with the toilet brush and all gone. I'm even more excited than when I got my last washing machine 
Unfortunately from my very brief googling I think you can only bulk buy it, not checked amazon or eBay though.