If you are absent from work for three days or more because of illness, you need to complete form SC2 (or SC1 if you are self-employed). These are available from reception or your employer. For absences of seven days or more, you need a certificate from your 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 recently been seen by a doctor.
Notes which are issued for reasons other than this fall outside the NHS contract and will be charged for.