Flea - we also had this trouble with our 3yro boy. He ended up really constipated and then wouldn't go because it hurt. I made up a huge reward chart which I stuck on the bathroom mirror so he could see it whilst sitting on the toilet. He's a big Thomas fan so each time so the rewards were all Thomas related. I divided the chart into six columns and in each one of the first five I stuck a new pair of Thomas undies and a little Thomas book into a plastic bag (he could then see but not touch). There was a big number over the column. Into the sixth column I put a Thomas DVD. Each time he did a poo in the toilet he chose which book/undies he wanted and he put the new undies on and we read the book (amongst much fanfare). He also got to cross out the number. I had to do this in conjuction with using parachoc (chemist) which is a stool softener, not a laxative. Made it all come out a bit easier. By the end of the chart he was then OK with using the toilet.

Good luck.