Visit 1 This visit will last around 30 minutes. The researcher will confirm that the potential participant has understood the information sheet and answer any remaining questions. The participant will then be assessed for eligibility against the criteria previously set out. If eligibility is confirmed, participants will be asked for details of current medication use including contraception, smoking status, and significant past medical history. Height and weight will be recorded. Participants will complete the Hospital Anxiety and Depression Scale and the Patient Health Questionnaire-12. These questionnaires measure psychological traits that have been associated with symptom response in IBS and so will be relevant covariates.