Aggression How do I stop this?

Did you introduce them in neutral territory?

Yer, neutral territory. Also while she was separated from them I kept their cages next to each other and they would all touch noses. I would also switch around their cozy's so that her smell was still in the main cage.

At first she constantly attacked all of them so I had to separate again. I then thoroughly cleaned the cage and put down fresh fleece and bathed them all together. That at least has calmed her aggression, though not completely. They all lived together before her operation quite happily, though it was difficult at first to inter-great Chewy (the one being bullied) into the group around six months ago (I took her from a family who had lost interest).

I'm really at a loss here.

Is it possible she's still in pain from her surgery?

The surgery was just before Christmas to remove a cyst on her back. It was only just under the skin so was a minor surgery, its completely healed now, I cant even find the scar anymore.

The only thing I can think of is that while she was separated she was in a smaller cage (old pet shop one so too small). This has caused her to get a little bigger, and she has always been very strong so now she is able to pick on the others without them being much of a threat to her.

Gosh, I'm no expert, but it does seem serious that your dominant pig is causing Chewy so much distress! Is there any way you can expand your cage to either 3x6 or 2x7? Maybe the girls just need a bit more room? I do hope one of the experts sees your thread and can chime in with some advice! Good luck to you and your piggies!

How long were they back together before you separated again?

I think that you should at least try to expand(It solved all of my problems! ).

How much does she weigh? If you is overweight you may want to put her on a diet.

They were together for around half an hour before I had to separate again. She was so violent, she drew blood from one and launched a full on attack on another, I threw a tea towel over her to break up the fight. They were terrified.

As for her weight gain, she isn't overweight, just a little bigger and now that she has more room for exercise I'm sure she'll get back to normal, plus they get plenty of floor time.

Perhaps expanding is the way to go. I cant go to a 3x6 as our flat is teeny and getting past them is awkward enough. I could try and convince BF to let me go to 2X7