As Si and Daz have said, there's nothing at all wrong with shooting from a seated position even if you're sat in the middle of a field. As has been said, so long as your movements are slow, you're aware of where the rabbits are looking as you move, you keep quiet and down wind, you'll be fine
I normally shoot seated as I can't shoot from a prone position because of a shoulder injury and I've never had too many problems as a result. Follow Si's advice mate, and you'll be fine.
Cheers.