Sorry I can't offer any advice, but we've got similar issues with DC who's also 20 weeks. Used to sleep until 5 or sometimes even 6.30am, now waking up at 2 or 3am again, not hungry but just lying there awake and gurgling away very loudly! I'm desperately hoping it's just a phase that will pass soon...
More info [http://www.askmoxie.org/2006/02/qa_what_are_sle.h tml here,]] here,hereand here.
If your LO was a good sleeper before, she'll probably go back to being a good sleeper without you doing anything. I just fed every time DS woke, but he was just waking frequently, not being up for long periods.
He has been awake for 1.5 hours in the night very predictably for later developmental spurts though, and although he does get into frequent waking habits, the being awake for long periods always disappears by itself. (But he's generally not a good sleeper so needs help to re-learn once the spurt is past).
Moxie, whose blog was one of the links, speculates as to which is worse - your first child when you're in despair that it'll never pass, or your second, where you have no patience because you know it does pass and are just frustrated that they aren't getting on with it sooner.