Warning: Don't pour fat down the sink! But if you do, try this. I'd blocked the sink pouring fat away from homemade oxtail soup and didn't even think about the consequences. I tried boiling water, bleach and then an alternative drain unblocker which all failed to budge anything and I was starting to panic. Your manager in store suggested this and after scooping out as much water as possible, I poured this in and went out for the evening. When I got back, the blockage was gone! It did say 'leave overnight' but it was gone in 5 hours. Highly recommended - saved me getting a plumber in!