Hi Lucy,
I got lucky because one of my beta readers raved about my book to the Editor in Chief at Oceanbooks. Remember that if you want your book to be a commercial success, you have to listen to what commercially successful writers say about your writing. That doesn't mean you have to adapt your style to suit their preference. They should respect that you have your own way of writing.
Being true to your own voice is essential. It's what makes you unique as a writer. When I first started writing I wanted to be Ian McEwan and tried experimenting with elaborate metaphors, which made my writing contrived. Say what you want to say and don't try to be like anyone else. You'll find you naturally use figurative language so if a metaphor or simile pops into your head, write it.
Once you've written what is as good as you think you can make it, send it off to be critiqued by someone who is an accomplished and published author. In your case your tutor at NZWC. He or she will make sure it includes all the elements of good narrative and revise in the light of their feedback.
So, your book is complete? Or you think it is. The next step is to pass it around for peer review. By all means let your friends have a read, but really, you need someone who will give an honest and fair appraisal. Send it to writers groups. These people, however, are not editors, so don't allow them to change anything. Listen to their suggestions but only act on them if several readers are saying the same thing, or you agree. Remember, however, that to have passed the Advanced Writing Course, your book needs to be publication standard so you are now an accomplished writer.
The next step is to get a professional editor to work on your book. After I had my book accepted at Oceanbooks, thinking it was as good as it could be, the editor went to town on it, but really going through the minutiae of ensuring that the best words were written in the best order. By and large, I took my editor's advice but you have to be the judge of whether the editor is allowing you to be true to your own voice. My editor had a penchant for using the word 'ruefully' and knowing my characters better than she did, I knew the word didn't fit. Not to say that the initial word didn't warrant an alternative. She also slushed up some of the romantic scenes and personally I thought she turned the characters into Blake and Chrystal from Dynasty. The editor can insist you make the changes they suggest and refuse to publish if you don't, but be prepared to discuss it with them.