Ok clearly I can't stop looking online but I just came across a really detailed timeline of what happens after an embryo transfer - I can't copy and paste on my iPhone but the points of note were:

5 day blastie
6 dpt - HcG production starts
9 dpt - enough HcG for a +ve

3 day
8 dpt - HcG production starts
10 dpt - enough HcG for a +ve

Obviously this is to be taken with a grain of salt