I'm sorry I have no advice but just wanted to let you know its not abnormal for her to do this. Matilda does it as well, she will hold on her poo and then ask for a nappy just before bedtime and do one in the nappy. It shows amazing control but it is annoying. We have tried talking with her about doing a poo on the toilet & she said "can't do it mummy" so we just decided to let her take her time. We did tell her that her first poo on the toilet would get her a chocolate frog for incentive but nothing yet.

Today she did a fart on the toilet & ran out & said "I did a poo!" but when I went to check I asked her what happened & she jumped on the toilet & farted & said "I did a poo!" so I had to explain farts LOL...