Should I retake after a few months ? (1st attempt : 680 )

Hello guys !

I'm coming back here for a question regarding retaking, or not.

I got an average 680 ( q44 v38) in march 2013. My goal is to apply to a Msc finance in the UK (Msc finance warwick, Msc A&F LSE, maybe cass, I haven't really looked into it yet).

The problem is that I'm going to start a very demanding study program in september (program in apprenticeship. 3 day in a company/2 day of classes). I know I won't be able to work on my Gmat after the 1st of september. I have basically until the end of the month to get back to my level from March and do even better (at least 700 with a good quant).

The good point is that I have nothing to do in August, only 3 days in London. Apart from that I can use every day to work hours on the gmat.
I have some good reason to believe that I could do better this time. A week after the test I was reaching Q39 and a Q47 in the gmat club tests. With some good training I think I could do better than q44. My last mock exams were around 720.

What do you think ? Are 20 days enough ? Is it worth it ?

Thanks !