It will be below Zero as we are doing the Rara Lake trek.
Well that rules the 500 out then.
Comfort temp under the E13537 rating scheme means a standard women, lying in a normal sleeping position (ie you don't need to be huddled up), should sleep in comfort. It assumes you're in tent, on a properly insulating sleeping mat and wearing a single layer of thermal underwear. As a women that is the temp you should look at when buying a bag. Men, particularly larger men, can looking between Comfort and Limit.
Bearing in mind that everyone feels the cold differently, my advice is to leave a healthy buffer between the Comfort temp and the lowest temp you'll face. So you need to know what that temp is. "below zero" covers a lot of ground.
I would have thought the 700 would be at the minimum level of protection you'd want, quite possibly you'll want more. I just googled Rara temperature and according to Accuweather it's -6C tonight which would rule that out too.
Check out:
http://sleepingbags-cumulus.eu/uk/categories/sleeping-bags - good quality, good value and go as warm as you need. For instance the Alaska 900 has a comfort temp of -10C for about £250. Alpkit also have decent value bags. If money is no object then there is an almost infinite range out there. I'm only looking at the lower price range.
But without knowing the lowest temp nobody can tell you what you'll need. You don't want to go for something way over the top either as you'll boil in say a -30C bag and might not sleep a wink.
Also you've not mentioned accomodation. A teahouse is a different to a tent.