Hi Clare,
My BP was really low a few weeks ago (80/50) and then went down at my 24 week visit to 70/40, I was told not to worry about it as that can happen in the middle of pregnancy and then come up again- there is no risk to you or the baby as long as you are feeling ok ( not dizzy etc ) and drinking enough. Mine has come back up now at 25 weeks to 90/60.
Hope that helps!