I was quite concerned about my OTP back to back following navylad's post. However, having done it last week I can confirm that the earlier posts which said it was easy were, in my experience, true. Get off the plane, follow the signs to transfers. Tell the person on the transfers security check that you are flying back to London and came to Bucharest as the fare was so good (no point lying!), he'll send you upstairs for a boarding pass/passport check, and then when you go back down you'll be let through. Less than 10 minutes from leaving the plane to being airside. YMMV, but I wouldn't hesitate in doing it again (and it was certainly easier than getting the last flight over and the first flight back!).