The community of microbes living in the gut is called the 'gut microbiome'. Changing this could be an exciting new way of treating people living with ulcerative colitis (UC). UC is a type of inflammatory bowel disease. It affects 4 in every 1000 people in the UK. UC causes severe episodes of inflammation leading to bloody diarrhoea. The gut microbes of people living with UC are different to those in healthy people. This may be part of the reason people with UC have a more inflamed gut. Prebiotics are types of fibre in the diet which help feed the positive microbes in the colon. Eating them can change the make-up and activity of the bugs which live in our gut in a good way. The goal of this clinical trial is to test the effect of a type of prebiotic called a human milk oligosaccharide (HMO) on the symptoms of patients with UC. The main questions it aims to answer are: * Can a prebiotic improve symptoms for patients living with UC? * Can a prebiotic improve the gut microbiota of people living with UC, and improve markers of inflammation, metabolism and immune function? Patients will take a sachet containing either the prebiotic or a placebo for four weeks, then swap to the other sachet. The trial will be double-blind and randomised. This 'crossover' design means patients act as their own control, which is important in gut microbiology studies. The prebiotic's effect on patient symptoms, metabolism and immune system will be measured. The investigators plan to recruit 44 participants over 18 months. Their urine, blood and stool will be tested. This project will be the first 'bench to bedside' study into the use of prebiotics in IBD. The treatment in this project is rooted in gut model studies. Different prebiotics were tested in the lab to determine which was the best to use for the trial. This 'lab first' approach is a first of its kind.