So there's a girl I work with I like a lot, there's been something between us for the past 18 months or so.

However last year she met someone, and at the start of this year the flirting between us stopped - for obvious reasons mid way through this year she moved in with him.

However the past two months now she's started flirting with me again and I've noticed her checking me our and looking at me like she used to.

Question is can she really be in love with her BF is she's starting this with me again?

Flirting in isolation should not be read as the green light. I am an natural flirt myself and its just a means of complementing another person . You need to look for other signals such as playing with her hair  in your presence , nervousness when you are around and eye contact. If you need to learn how to read signals then there are plenty of videos on You tube that deal with body language and eye contact .I just feel that you may be looking too much into this but I could be wrong .

If you do find that she is into you then you will need to ask about her existing relationship . Encouraging her to cheat isn't a good idea as you will always be second best and the relationship will go nowhere . The only way you will get the answers is to ask and talk.