Sorry Eloethan, I have only just seen your question
It was a real challenge trying to navigate the website (link above) and find the answer, but it is there, tucked away.....
If you do any work in a self-employed capacity during your MPP, then such work will not affect your SMP
However, if she gets pregnant again, and is only working in a self employed capacity, she will not be entitled to SMP.
She will have to claim Maternity Allowance instead and not earn anything on top of that.
Hope that helps