A 70-year-old woman on ICU has been treated for hospital-acquired pneumonia with piperacillin/tazobactam. After 5 days, she develops recurrent fever, abdominal distension, and profuse diarrhoea. C. difficile PCR is positive and toxin EIA is positive. Flexible sigmoidoscopy shows pseudomembranes. She remains haemodynamically stable. According to NICE NG199, what is the first-line treatment?