Thanks for taking the time to reply but we have tried the suggestions so far.
When she stops she just drops on her belly and steadfastly refuses to budge. Treats and coaxing do not work and the only way to move her with the lead would be to drag her. It's almost come to this but it wouldn't do her or me any good to go down that road!
It doesn't seem to make any difference to have the other pooch with us - he is very confident and loves to go out but she just seems to decide she has had enough and thats it. In fact she has decided since that she doesn't want to leave the house either.
The last thing I want to happen is to be dragging a dog like a stone behind me or letting her get away with dictating what happens and when. However, I am not sure if this behaviour is a reaction to some bad experience or just plain stubborness and bossiness on her part so the more suggestions the better as nothing is working so far.