I notice you're asking for content related to "caustic soda" and "Sainsbury's."
Look for branded drain unblockers (e.g., Mr. Muscle) in the household cleaning aisle. Always wear gloves and goggles, ventilate the area, and keep away from children/pets.
If you need content for informational or safety purposes, here is appropriate messaging: Caustic soda is dangerous if mishandled. It can cause severe burns to skin and eyes, and damage surfaces. Sainsbury’s advises customers to follow product labels strictly. For drain cleaning, consider enzyme-based or less hazardous alternatives available in-store.