Toby is 42 years old and returns to the GP with a 2 month history of generalised itching and fatigue. He denies any abdominal pain or skin rash. There is no recent travel and no one in his household has similar symptoms. He has a background of Ulcerative colitis which is well controlled. Your colleague saw him 2 weeks ago with the same symptoms and ordered a Full Blood Count, kidney function tests and liver function tests. All results are normal except for a markedly raised alkaline phosphatase and mildly raised bilirubin.
His observations today are stable and you note hepatomegaly which was also observed by your colleague.
What is the most likely diagnosis?