The aim of the study is to investigate how dietary fibre combinations affects gut physiology, particularly colonic gas production. Comparisons will be made between a single fermentable fibre (inulin), a non-fermentable fibre (psyllium) and a combination of the two. The study will also explore differences in response between diarrhoea-predominant and constipation-predominant IBS (IBS-D and IBS-C) respectively.