Bumper - feeling your pain you poor thing. With 2 in the same bedroom it's a miraculous rare event when they both sleep through a whole night. It's so bad I'm actually happy when the first peep I hear from them is at 6 a.m. . I hear stories of children sleeping till 7 or even 8 alas, not in this house!!
Anyway, take a deep breath, and think through the list of things which could be waking your LO. Hot/cold? Growth spurt? Scratchy bodysuit? NIghtmare? Teeth? Tummy ache? Have a good listen for a few minutes first when she wakes up next time and try to watch unobserved if you can as well. I'd try getting out in the "daylight" (sorry this British weather leaves something to be desired!) for a good spell during the day. Limit daytime naps. Give plenty of time at bedtime for winding down - read a gentle story and keep TVs off, lights low, and sooth her to sleep nice and slowly. Make sure it's nice and dark in the room. Then, cross fingers and don't rush in straight away. Don't rush to pick her up either, try limiting the picking up and soothing by touch or whispering instead. HTH...
wish I had a cute name for DD. at home we call her cupcake. or mouse. but baggins and jamlet are much cuter