At baseline, subjects will be admitted overnight for a 24-hour endocrine profile whilst on their standard therapy. Subjects will attend the study site in the morning and have 17-hydroxyprogesterone (17-OHP) and androstenedione (A4) levels assessed at 15:00, 17:00, 19:00, 21:00, 23:00, 01:00, 03:00, 05:00, 07:00, 09:00, 11:00, 13:00 and 15:00. Safety laboratory tests, a DEXA scan for body composition, and height, weight and waist circumference will be recorded. Subjects will then be randomised to Chronocort® or to continue on their standard care. Randomisation will be stratified by baseline treatment: