A bit of hen psychology needed here please.

Hi and welcome.
Sadly you will always get one that is at the bottom of the pack and she is it. Some hens are happiest on their own just looking on but never joining in and this may be the case for her.
To get her re-introduced if this is what you really want to do then you could try placing one of the less aggressive ex batts in with her for a week or so. Make sure they can all see each other but not get to touch. then when they have formed a bond you could try introducing them back in with the others or even try adding another one or two.
The sorting out of the pecking order can take several weeks and if you think about their previous life you will realise why ex-batts can be quite aggressive to each other - they have had to fight for food, water and space.
When you do re-introduce her make sure you have loads of feeders, drinkers and hiding places also plenty of treats (if you scatter corn they can't guard all of it can they  . If they are in a fixed run have plenty of things to avoid boredom setting in  .
Where abouts in South Lincolnshire are you?

Thanks very much for replying so soon, great to know all those out there keeping an eye open!  Im only just in Lincs, about 100meters from the west Norfolk border, near Sutton Bridge.  Your reply is of great help too as I know that introducing only 1 to 17others could be a reciepe for carnage.

On a different subject, I was interested to read about bumblefoot as I had never heard of it.  Last year we had one oldish hen which developed such a growth on the side of her leg which grew to a length of about 6cm very quickly.  She always seemed unfazed by it as her general health was good, having said that she died a few months later.  I have photos of this growth if it might help the forum?

Thanks very much for replying so soon, great to know all those out there keeping an eye open!  Im only just in Lincs, about 100meters from the west Norfolk border, near Sutton Bridge. 

not too far from me - I'm about 10 miles north of Spalding  .

and yes a photo of bumble foot could be useful thanks

Kenilworth is spot on. We had two Welsummers which came as chicks and were bottom of the pecking order (despite their size). When we introduced four more birds they made very effort to be aggressive and assert themselves. Really obvious behaviour from a human viewpoint.
We didn't seperate the newcomers but gave them plenty of space to keep away from the original group so there were only one or two actual confrontations. Little by little they joined in and even the most shy now perches with the others. It has taken about 15 weeks but they now behave as one group. HTH