Poppagator - interesting to hear your view on diverticulosis. Both Manitas and my mother have this condition, and both have been told to eat lots of roughage.
(For those who get confused, diverticulosis is the condition of having little pockets in the large intestine, diverticulitis is the condition of inflamation in those little pockets).
For the IBS, this site has some useful tips & info about it. It was recommended to me by a work colleague who is an IBS sufferer and controls hers by diet (she's a vegetarian) and takes those friendly bacteria yoghurt drinks to aid the flora in the gut that make digestion a happy experience.