The HMRC Guide E14 states,
“For spells of sickness lasting four to seven days, you may accept self-certification verbally or by letter, or they can use form SC2 for self-certification, or your own equivalent form.
Incapacity lasts more than seven days
If your employee is sick for more than seven days, you can ask them to give you some form of medical evidence or a fit note from their doctor to support payment of SSP.
As an employer, the decision on whether or not evidence of illness is required, and if so, what evidence is acceptable, ultimately rests with you.”
It’s up to you if you want the evidence or no but you can download the SC2 here,
http://www.hmrc.gov.uk/forms/sc2.pdf