I did a little futzing here as well:
If your book has Wattcode xyz, simple download the URL
http://m.wattpad.com/offline/Wattpad-xyz.jar. Use your favourite zip-like program to open the archive, and you will find a series of text files of the form "xyz-10000", "xyz-20000", etc. Concatenate these to get your book.
Or you could just use Neill's approach above.
And these have no formatting at all. Plain text only.