We can't tell for sure from your photo, but wonder if it is Asiatic Dayflower. It's flowers are a purplish blue, but very notable for only having two petals. Please put this name in a search box on google and look at images to see if they match. 
Otherwise, please give us a photo/description of the flower, as well as a photo in situ which clearly shows the leaf and growth pattern.
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Thank you for your speedy reply. It is indeed mouse flower. Invasive is the word! Thank you very much for your help.

Glad to help.
Just fyi: We are not sure why the first photo was attached here, but that looks to us to be the perennial plumbago. It also has a blue flower, and when happy, can take off with great speed as well.
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