Ask her out for lunch as a means to getting her alone to talk to her. A simple request to take the lady out to lunch to chat is probably easier for you and her.
She may well be wondering why you haven't asked her out yet so to asking her out now may get her flustered and catch her off gaurd.... whereas a lunch is a chance to communicate without interruption.
Over lunch tell her you actually wanted to ask her out but wasn't sure how she'd react. Tell her if she wants to just be friends, that's fine but if she'd like a night out to see how you get on with each other outwith the work situation that'd be even better.
Be honest with her and remind yourself that you have got a lot to offer.
As Tennyson said. Tis better to have loved and lost than never to have loved at all.
As ABBA said. Take a chance
Good luck and keep us posted.