If you read the information on Sensible's site again you will read the following, concerning the gall bladder flush:

The use of Epsom Salt
If the Gallstones are so packed that they have difficulty moving, Epsom salt, which dilates the bile duct, may be helpful. This usually happens to someone who is doing the first flush. Nothing may come out at all. In this situation, you may have to repeat the olive oil/lemon juice procedure and eat one more day of vegetables. However, Epsom Salt solution, prepared by dissolving 3 teaspoons of Epsom Salt (magnesium sulphate) in 1 cup of warm water, should be taken about one-and-a-half to two hours before repeating the olive oil and lemon juice procedure.

You may need some as a precaution even if you don't use it as part of the procedure. I don't use the Epson salt anymore but I have flushed when a stone has become temporarily stuck so I know from experience that the Epsom Salts needs to be nearby.