A 25-year-old man with known Crohn's disease affecting the terminal ileum presents with increased stool frequency (6 times/day), abdominal cramps, and fatigue. He is currently taking mesalazine granules. Inflammatory markers (CRP, faecal calprotectin) are significantly elevated. According to NICE NG129, what is the most appropriate initial treatment to induce remission of this flare-up?