I went to a great new restaurant on Saturday evening: Franklins on Kennington Lane. It's SE11 rather than SE1 but it would definitely be worth the short journey. The food is really decent, inventive modern British, there's a good wine list and the service is excellent: friendly without being intrusive. It's just opened and I'm sure it will be a great success.
Oh yeah, for tapas both Mar I Terra (just off Blackfriars Road) and Meson Don Felipe (the Cut) have something to offer. Mar I terra has a little terrace tucked away amongst the railways lines (better than it sounds!) and Meson Don Felipe does live Spanish guitar music through the night.
Welcome to the SE1 forum. I assume you have no connection!
I have been to Franklins and, personally, consider it overpriced. I had a starter which was a tiny piece of smoked eel, for £6.50.
I also have friends who have been insulted when they dared to suggest to the owner that the service was too slow and that the starters were poor value. They were only trying to make helpful suggestions so that it could improve.
The owner had the cheek to say that it was "central London prices". The problem with that is, living where we do, we have the choice of the whole of central London to eat in, and they have to be good to get our custom.
I really like the restaurant on top of the Tate for a special occasion. Fantastic view (and the food's not bad either, and they have bottled Dunkerton's cider, and the menu is flexible with good supper/brunch options as well as the full 3-course thingummy).
Blueprint Cafe is also nice (and they have been doing some reasonably priced deals recently, which I think you can find if you search through James' list of SE1 restaurant special offers on this site).
And the mad French fish restaurant just Sth of E&C (on the turn off to Kennington - The Lobster Pot, I think it's called) is also unique and good for a special occasion imho.