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.