A 40-year-old woman with known Crohn's disease affecting the terminal ileum, currently in remission on Azathioprine, presents with increasing fatigue. Blood tests show: Hb 95 g/L, MCV 108 fL, Platelets 150 x 10^9/L, WCC 4.0 x 10^9/L. Blood film shows macrocytes and hypersegmented neutrophils. What is the most likely cause of her anaemia?