Emma, my ob recommended not starting before 6 weeks post partum. You don't have to wait for a period (which is just as well because it can be over a year before AF returns when bfing). I tried it twice, once after each baby, but it caused me to bleed all the time so I have now had a Mirena fitted. Neither boy seemed to notice any difference with my milk, although I have heard it can happen.

Personally I am way happier with the Mirena. It is a lower dose of hormones than the mini pill, but is localised and no risk of forgetting a pill, so probably safer. And it is safe while bfing. It can be removed at any time and have your fertility return if you want another child.