I'm part way into writing a novel and after looking back over the work I've already done i'm seriously thinking it needs to be rewritten to match some of the changes I have made to the plot over the course of writing the book. But I wonder if I should leave this till I finish the book or start now and rewrite what I already have?
I would definitely suggest finishing what you've already started before going back. The current part that you're writing is still in some ways connected to the first part, and your mind is still following that train. If you go back and rewrite the first part, it will not match with the rest.
It would be a pain to have two parts of your story that are off and then try to catch them up with each other.
Finish what you started. Go back and rewrite it all. Rewrite again.
David Eddings, author (with his wife) of the Belgariad series, suggested something along the lines of:
"Write, write, write, write, until you can't write anymore. Then take your drafts, throw them all in the fire and start your novel."
Unfortunately he didn't follow his own advice, instead publishing the stuff as the Mallorean series.