Do I need one? For the first four days of any illness you do not require any form of certification. It is for you to decide if you are fit for work. For the next three days (including Saturdays and Sundays) you must fill in a self certification form which can be downloaded from here and submitted to your employer.
After the first seven days you will need a doctors certificate that can be arranged free of cost. If this is your first sick note you will need to book a telephone consultation. If you have already had a sick note for this period of illness you may not need to see your doctor to receive a subsequent sick note.
Private Sick Note: Some employers or insurance schemes will ask you to provide a sick note even if you have been ill for less than 7 days. These are called the private sick notes but the surgery will charge you a fee of £25 to provide you with one.