What is in those sink unblocker things?

[BertieBotts]

A friend was helping me clear up last night and must have poured away the fat from cooking down the sink before I noticed as my sink is now blocked. Last time this happened I bought some of that sink unblocker liquid which worked but I'm wondering if it's just a gimmick and something else would work. I have got some bleach, but thought I should check first in case I do end up just getting the sink unblocker and it reacts badly to bleach or something.

I think it's caustic soda because it says corrosive on the back of them. I'd try a few kettles of boiling water and soda crystals first. Aldis doing a sink unblocker for £1.89 that's £3.05 in wilkos. Caustic sodas bout a pound but you have to be really careful with it.

I've blocked sinks by putting it down without water and bleached the wood work top by splashing it about.

Soda crystals and hot water are effective, as stated.