If it's a one-time download, as opposed to something you're trying to automate for repeated download, you could also just use dotepub (
http://dotepub.com/).
This, combined with the 'automatic adding' feature that calibre added recently makes it extremely easy to go directly from a web page article to an epub in calibre. (I literally just hit the dotepub bookmark on my toolbar when I want to save an article and 20 seconds later it's sitting in my calibre library.)
Only caveat is that the entire content needs to be on the page you're viewing - most sites have a 'single page' or 'print version' option, but some don't.
ETA: I just realized that the file is already an epub embedded in a website. Not quite the same thing.