Update: OK so I couldn't make it in for lunch time today but I saw her in the afternoon. Things were going well I thought, I managed to slip her my personal number 'just in case' and she asked me what I was up to for the rest of the evening...
Of course I didn't want to seem available so I told her about a friend who'd been injured and has been housebound for a few weeks because of it so I was going to take him out to cheer him up (90% true, I took him last out night).
Anyway she told me exactly where she was going with her friends including time, place etc but stopped short of asking me along. It seemed like they were having a girls night out anyway and I had an injured friend to meet afterall so I dropped it, but later I asked her when she got off work and if she wanted to go for a quick drink on the way home so we could chat a little better out of the office environment.
So she said she couldn't cause she had these plans but 'some other time'...urgh. I had no Plan B as I know little to nothing about her but experience tells me she's saying no... Then again she did tell me about the plans first and for all I know she lives far away and needs to go back to change and eat supper etc. She also gave me a big smile as I left and said she'd be in touch tomorrow but I'm not sure whether she was in customer service mode or not.
Think I might have fucked this one up but I'm not sure

How do I proceed from here guys?