You just need to write a short letter when you send the sicknote in late explaining you couldn't get an appointment in time - of course they understand that (it happens all the time!)
If you feel you can't attend I'm sure it will be fine as long as you let them know (but tell them in no certain terms that you were not able to attend)
Bruno - it is not hard to get through to ATOS, I have phoned them many times, please check your facts before posting advice to others on things like this and possibly worrying them unduly.
I understand your frustrations, completely, but please don't let your experiences cloud your advice to others. Cheers!