The main thing for me was that the HV helped me see that what I was asking my DD to do (ie come out of the towel cupboard where she would retreat with a nappy on to poo) and poo in the toilet was just too much of a leap for her, regardless of how many shiny things I tried to bribe her with. She said lots of kids dont like the feeling of space beneath them when they poo and for whatever reason they feel more secure pooing in the nappy. So her suggestions:
To completely de-stress the toilet experience itself with lots of relaxed 'practise pooing'- ie sitting on the toilet for a certain length of time eg while you read a story, which absolutely no pressure for anything to happen poo-wise. She suggested after tea pre bath, as the gut contracts after eating, and it was an unhurried part for the day for us. This sitting was rewarded with a sticker and lots of praise. DD was then given a nappy to go and poo.
(Our situation also involved edging DD closer to the loo so in addition we had to very gently get her out of- but next to the towel cupboard, nappy on, then closer and closer to the loo)
Then once you are up and running with practise pooing she suggested giving blowing games on the loo during that period- bubbles and balloons or harmonica etc as what you are after is an accidental poo really so you can almost pass out with praise and excitement etc.
Then we moved to - 'practise pushing' where I put my hand on her tummy and she pushed say 5 times before she could get off.
Eventually- by keeping her poo soft with fruit juice and lactulose etc she accidently went. She cried briefly, I more or less tap danced, she got her shiny things and made a decision to poo every night. If she hadnt made this decision we would have just continued as before.
She also suffered hugely from constipation in the year before all this and I think that can really contribute to poo issues- often in a cyclical way as they withhold through fear and then get more constipated etc. We used lactulose every day as well to make sure that she didnt get constipated at all when we were trying this. Stay patient- it will come right!