Wont poo anywhere other than in nappy- when should I start to panic?

[kbaby]

DD is 3 and shes been dry day and night for almost 9 months. The problem is shes always been a bit secretive about pooing and never went when we were out and always hid when she went. She would insist on a nappy on to poo and we went along with it hoping she would outgrow it. Thing is she hasnt and still insists on a nappy. This week I told her we didnt have any and she had to use the toilet. This meant she held it in for 4 days and tantrummed etc until today I gave in and put a nappy on her today.
I dont want her to hold it in and make herself ill but yet I cant understand why she wont use toilet/potty etc. She said when she sits on the loo that it wont come out. MIL thinks its because she goes standing up and takes ages to go ahd therefore shes too impatient to sit on the toilet.
Should I just give in or get tough and make her go?
I dont want a 5 year old who still needs nappies.
Anyone have any experinece of this.

BTW ive tried putting nappies on the potty etc.

Have you tried cutting a hole in the nappy? I can't remember where I read this, but if I can find it in the general clutter of my home in the next day or two, I'll be able to substantiate that a bit

We cant get her to even sit down and have a poo. She takes ages and insists on walking about and playing.

bump

My ds would only poo in his nappy as well. I used to take him to the toilet once he had done one, and empty it down the toilet and wave it bye bye. I also sat with him in the bathroom and read him a book(s) to encourage him to sit and poo. Eventually I got him to poo on the toilet without the need of a book and now he goes as soon as he feels the urge. Just be patient. Ds was 3 1/2 before he mastered it (and now sometimes he has accidents - usually when he's watching something on the tv or playing outside and leaves it too late).
If she doesn't like listening to books, how about singing a few songs whilst sat on the loo?
Also, what type of toilet seat for her have you got? I ended up getting one with handles as he felt much safer with this one.
HTH