A 28-year-old woman presents with a 6-month history of intermittent central abdominal pain, bloating, and a change in bowel habit, alternating between diarrhoea and constipation. She reports that the pain is often relieved by defecation. There is no rectal bleeding, weight loss, or family history of bowel cancer. Abdominal examination is unremarkable. Full blood count, CRP, and coeliac serology are normal. According to NICE guidelines, what is the most appropriate next step in management?