For the first week of sickness you do not need a sicknote from your doctor. You can certify your sickness using a form, called 'SC2' available from your employer, or the benefits agency.
After the first week...
After the first week you will need a sick note from your doctor. You will need to book an appointment by contacting reception.
Private Sick Notes
Unfortunately some employers insist on a sick note from a doctor even if you have been off work for less than 1 week. Providing a sick-note under these circumstances is not a legal requirement, and is not provided for by the NHS. Providing these notes takes your doctor away from providing valuable NHS services and as a result we have to charge for issuing private sick notes.