I would phone the g p surgery, explain the situation and emphasise that it is imperative that your father receives the injection on the given day, they should have a letter from the hospital informing them of that. Our surgery is very good about getting the drug( prostap) in and also arranging the injection. Once when I rang a bit late they sorted out for early opening and the nurse to give it to my husband on the day that was stipulated. It is so important that you be assertive about this. From that time since I have counted out the days and marked it on the calendar, even highlighting to make an appointment a week or two before the due date, once he gets into a routine it should run smoothly, good luck