Hi Eve.
From my experience, each agent/publisher will ask for its own specific thing, whether it be the first three chapters, a certain amount of words, a short synopsis etc. The important thing is to be clear about what they are asking for and giving them exactly what they want! Sadly it is very much a seller's market and us struggling prospective authors have few cards to play!
If it helps, I have a blog that I started to chart the progress or otherwise in my attempts to get my novel published. Check it out if you get a mo!
http://tollesburytimeforever.blogspot.com/
If there is any way I can help please tell me. A success for one is a success for all!
yours in hope,
Stu