Hi Emma,
The cocker we had in the summer found a bone on our common, and got really awful runs. As she wasn't ours, and was not her normal self, we took her to the vet who gave us some powder to put on her food. We had to starve her for a day and then she had to have boiled chicken breast and rice for 3 days.
She was fine by the second day and of course was eating better than we were!
The powder may have helped, but it may have been the plain diet.
Good luck,