Concerned about returning to work following maternity leave

I am currently on maternity leave and I am due to return to work in November. Everything was fine until I went into my work to discus my return, I wanted to return early but on a part time basis. I spoke with the general manager and he just employed a new sales manager and I was never asked. The head count of the team was 5 however it had to be reduced down to 4 meaning that no one was on a temporary contract, my manager said he would sack someone for my return. 

After this meeting I raised a grievece which went through my regional manager. He told me that he would find out why I was never asked and that usually promotions are considered if you have hit your targets the previous months. Also he said my job is still there because as a business they can change the head count as they please. He said they will raise the head count back to 5 when I return then put it back to 4 when someone leaves. Also I asked about returning part time but he seemed to make this very difficult for me so I said I would just return back full time when my maternity ends. I feel when I do return that my job will be made very difficult as I have raised a grievance and they have a track record for sacking people on the spot which I confronted them about. I'm really anxious about returning. What can I do?