The NHS provides sickness certification only after seven continuous days of illness. If you need a sick note to cover this period of sickness you should complete a Self Certification (SC2) form which is available from your employer or on the HMRC website.
Please visit the Direct Gov website for full information about fitness for work following a period of sickness. After this period a NHS certificate may be issued but you will need to contact the surgery as it may be necessary to discuss your request with your GP.
If you wish to return to work before your sick note expires, you do not need another note from your GP.