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will ask about anti poo devices too.
You want one of these, or one like it, the right size for the pipe:
https://www.celticwater.co.uk/inline-backflow-valve-anti-flood-non-return-valve-110mm-160mm-200mm/As that one has a lever for total shutoff, you'd want an access hatch too, although it will still automatically stop the poo from flowing backwards when the lever is open.
My plan is to put one under the house when we do the new kitchen/downstairs loo but I don't know if this is compliant with building regs.
Did lol at:
Unfortunately we cannot accept returns for this product when it has been fitted.
You have a cellar?!?! How deep? We have 1.5m of drop under our downstairs... will ask about anti poo devices too.