I don't think they would take too kindly to you not turning up- best thing is to phone up tomorrow first thing, and ask if there is anyway the date could be changed. You could always say a crisis has come up, like you have to take your ill mother to hospital or something!! If it's easy for them to change, all well and good. If not, maybe you could explain to your boss that you tried really hard to get out of it, but they won't let you. Assure your boss that you will make up the time. Maybe even try and get into work after your appointment, and work late that day. You can't please everyone, unfortunately!!