How to dissolve grout

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While on holiday, my father-in-law very kindly finished tiling our ensuite. But from what I can ascertain any excess grout went down the drain. Ever since the drainage has been poor. I have tried Sulphuric acid based cleaners aswell as Sodium Hydroxide/Hypochlorite cleaners. Any thoughts, as I really don't want to resort to pulling the floor up and replacing the piping.