On Mon, 11 Dec 2017 06:55:34 -0800 (PST)
wrote:

On Monday, 11 December 2017 14:29:11 UTC, Rob Morley wrote:

And caustic soda (sodium hydroxide) a precursor of sodium
hypochlorite, also useful as a cleaner, and available cheaply £2
for 500g in Wilko. (B&Q website claims to sell it for half that,
but none of the nearby stores have stock).

I thought that was present in household bleach.

It's an "ingredient" so perhaps present as a result of incomplete
reaction?