DD1 didn't sit for ages - months after everyone else at the same age. She then didn't crawl until ~ 14months and then walked a month later. She was happy enough just hanging out...and was quite chubby so I figured it was just a bit harder for her to get moving!
I reckon Nelle is on the money - definitely worth a bit of practice before you get too concerned.
Every bub is different - certainly if you are concerned about it I'd seek a GP/paeds opinion just to check out things.
Good luck!!!