It is now not necessary for your GP to issue a certificate for the first 7 days of your illness after 3 days of absence.
A copy of self certification form called SC2 can be downloaded from http://www.hmrc.gov.uk/forms/sc2.pdf
Ask your employer for a self certification form which allows you to self certificate for up to 7 days.
If you are ill for more than 7 days you will need to see the GP for a certificate, please make a routine appointment.
If you require a sick note from your GP for insurance purposes you will need a private note and this will incur a charge.
Please speak to the Practice Manager about the details of this.