John Innes is fortified but I've seen a lot of growers in UK report successful outcomes using it. The mysterious element is this multi purpose soil. I may have missed a description of it. What is it exactly? From the sound of it you're using mostly this multi purpose soil and have added in some John Innes. May be some funky stuff in that bag, a little suspicious an extraneous plant of unknown origin is coexisting in the pot. What else is in there we know about?
As for water, let it dry out before next water and don't flood it. (Assuming your ph adjusting the water first).