There are all kinds of fussy solutions to this problem, like trying to change the pecking order by separating this chicken or that chicken but I always explore the root of the problem.
Space always takes care of so many problems with anti-social behavior and I always advise changing your current setup to free ranging. It provides so many other benefits for the life of the chickens, of which social interaction is just one small part, that it is well worth any money or time invested if you plan to have chickens for long term.
With space a loner bird can go off on her own and have room to evade other birds, roost apart, find food that isn't being guarded by other hens, get some peace.
If you cannot provide space so that your flock never has these bully situations, then it's merciful to cull the loner bird so that she is not constantly in misery. Remove her from the equation and put her either in the pot ~ or in someone else's flock to see if she will thrive better there.
I've been keeping chickens for a very long time and never had a bully situation...that is no accident. It rarely ever happens in a free range setup and one can quickly spot the real aggressor in the situation when the flock isn't all in one place all the time. Then you can cull for over aggressive behaviors.
So, if you already have them on free range and this bird is still getting actively bullied by the whole flock, it usually means they know something you don't and I'd advise to cull her. If she is getting bullied by a ringleader only and the others join in the chase, cull the ringleader.
Hi , thank you for the reply , all birds are free ranging , i just cant seem to pin point the one who is the agresser , i will look into this more as she is one of my best birds , i think it is her temperment as she is so placid and seems to be at the bottom of the order , a few chickens may need to go and change flock thanks