Welcome to the site.
My little boy was 4lb 10ozs when he was born, which for 33 weeks was smack bang on 50th centile, but he's been around the 9th for weight and length/height ever since. He's 3 now and has just gone into 12-18 month trousers!
I would try not to worry too much, if your little girl is healthy and has lots of energy, then she may just be small, or she may well catch up as she gets older. My friend has a little girl who was full term, weighed about 8lbs 6ozs at birth and she's a tiny wee dot now, very strong, full of beans, but just a bit little.
As long as the health visitor/doctor isn't concerned, and she's healthy, I'd try not to worry. I know it's easier said than done though!