Evidence shows that anxiety plays a part in IBS and can speed up transit time in the small bowel. In this study, the investigators hypothesise that since anxiety is a common feature of IBS, then fast small bowel transit is likely to be found in all subtypes of IBS and the difference in stool frequency and consistency in IBS subgroups are therefore likely to reflect differences in colonic function. The investigator wish to test this by measuring both small and large bowel transit times using Magnetic Resonance Imaging (MRI) and validate the results of the MRI with two methods currently used in clinical practice -The previously validated lactose-C13 Ureide breath test (for small bowel transit) and the standard radio-opaque pellet method to assess the whole gut transit (WGT) time.