You do not require a doctor's sickness certificate for any illness lasting seven days or less. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
If you are sick for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay).
A medical certificate, now called a 'Statement of Fitness for Work’ is available from your GP if required.
You can request a fitness to work certificate via eConsult
If you need to be certified as unfit for work as a result of the treatment provided by your specialist, they should issue a sick note when you are discharged from hospital or seen in the clinic. Please ask your specialist for one if you need it.
For more information see the DirectGov website