If you are absent from work for three days or more because of illness, you need to complete form SC2 (or SCl if you are self-employed). These are available at reception. For absences of seven days or more, you need a sick note from the doctor. These can only be issued if the doctor has seen you on the same day or the previous day, or if you were recently seen by a hospital doctor who has written to your GP.
Occasionally, a special note can be issued if you are unable to work because of chronic illness and have been seen recently by a doctor. Notes which are issued for reasons other than this fall outside the NHS contract and may be charged for.