Im in a pretty complicated relationship here, see my boyfriend was in love with my best friend for about a year, but she made it quite clear that nothing was ever going to happen between them; but he never really let her go. Then about 3 months ago me and him started speaking, and we really hit it off and then a month ago we started going out. However my best friend has just broken up with her boyfriend and is now all of sudden speaking to and paying loads of attention to my boyfriend, and I think he's starting to like her again. Its really upsetting me as I don't want to be his second choice as he can't get her, but I really like him. I don't know what to do, please can you give me some advice.

Tell her to back off! Ask him out straight, you will know by the look of his face or the way he answers if he does or not but I doubt he will admit it, so you will just have to make your decision based on how he answers i.e if he stutters or hesitates, or answers in a defensive tone he probably does. And when you evaluate the answer and conclude you were right then dump him because you will only get hurt... bad

Please do not judge him on the way he answers your question.

But yes, do ask him.
That gives you two choices.
A) Believe whatever he tells you, because that's what trust is.
B) Don't believe him, and break it off - because if you honestly think you're his second choice, and he might leave you for him, then your better off without him. Then find someone that will make you his first choice, and if it does actually happen remember to cut ties with your friend since no girlfriend steals a friends man!

He is your boyfriend, talk to him. She is your friend, talk to her, tell her to butt out. Some friend.