The IPP policy team referred the question to HM Revenue & Customs for further guidance and received the following response.
The point referred to talks about the individual not providing acceptable medical evidence in which case the employer can refuse to pay them SSP on the grounds that they do not accept the evidence provided.
However, where medical evidence is submitted or received late (and we are talking here about a certificate relating to day 8 onwards of an absence) the payment of SSP cannot be withheld. It might be that the late receipt of medical evidence could be, for example, because the employee was unable to get an appointment with their GP by the eighth day of incapacity or that their sickness has unexpectedly continued or recurred into the eighth day and an appointment with their GP has only just been arranged.
Where an employer can withhold payment however, is where the initial notification of an employee's sickness is outside the time limits set for notifying the employer that they are ill. In those circumstances the employer can withhold payment of SSP for the period of the delay if they do not accept there was good cause for delay. In these cases the employer should treat the date of the late notification as the first qualifying day for SSP.