You do not need a sick certificate until you have been off work for more than three days.  A self-certificate covers the period between three and seven days (including Saturday and Sunday).  If you are employed, you obtain this from your employer.  For self-employed or unemployed persons, you obtain this from the surgery, the post office or the DSS.  If you still require a certificate of illness up to seven days, a certificate can be provided on a private basis, but there is a charge for this service.