I've got a couple of bottles of supermarket own brand Toilet
cleaner/descaler (Waitrose and Tesco). Both contain Hydrochloric Acid

I know that I could got and get something cheaper from a BM/plumbers
merchants, but I have to get round to it. It's not like we use loads of
the stuff.
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I use the Tesco one and think it is really quite good. 74p for a 750ml
bottle.