I met with the gastroenterologist on Monday and the tests have all come back clear. He thinks I have a 'sluggish' gut and that is causing the constipation and then the diarrhea. He had prescribed a selection of laxatives last time, but they didn't seem to make much difference. He has prescribed a slightly different selection for me to try, assuring me that they are safe to take in large quantities (not sure I like the sound of that!)
I've been researching this a little myself and am going to include a probiotic (Yakult) in my diet to see if that helps. In addition, I'm going to try a home-made mixture of prunes/figs/dates/currants as well!
Hopefully, with my normal healthy diet and exercise, my gut will start to settle down and get back to normal.