Thank you for all of the advice! I will try walking her out in hand at the weekend and see how we get on! Every time you try and be firm with her she bucks or rears! It is my step-sisters horse and she has allowed her to take the mickey out of her so I am trying to undo all of this. I am not one to let a horse beat me so I will perservere!I'm sure we will get there eventually!

Thanks again!

My mare is the same, fine in company, but try to go on her own, she'll go, but bolt back the second she feels like it. >.<

You could try walking her out in hand first (in a bridle is probably best) maybe take her as far as she'll go, let her graze for a bit, then walk back so that she doesn't feel pressured. Once you can get a fair distance in hand, try it under saddle. So ride her out not very far, let her graze, then take her back. Essentially do the same thing under saddle to build her confidence, obviously rewarding her with your voice, and treats if possible for being brave.

Also when out on a hack with others, see how far you can increase the distance between you and the other horses, and whether she will let you guys hang back, or even go in front and see if you can get her to keep going forward and not worry where the others are.