Well nearly 2 weeks later...
I started with 13 platy fry, and now i have 9. 4 have mysteriously disappeared. I've checked inside the air powered filter and there wasn't any in there, so I'm guessing they either died and were eaten by the ghost shrimp, or they were just eaten by her. Either way i've removed the ghost shrimp and put her back in the main tank as a precautionary measure, so unfortunately when her babies arrive (if they still do) I'll just have to hope i notice them in time to transfer them to the fry tank.
I can't confirm that she ate them but i wouldn't recommend having a ghost shrimp in your fry tank to anyone in the future. The fry are 5 and 7 weeks old now, so they're not tiny anymore (some a lot smaller than others though) but like i said, I'm not convinced they weren't eaten.
Lesson learned! Fingers crossed my platys have more in the near future to boost my fry numbers again!