Rhianon, I think you need to very wary of this. By putting you on 3 months probation, they are giving themselves an out. If they really don't want you back - maybe they just don't have the work, or are worried about you having another baby - then they can just say you didn't meet their expectations.. They might already be planning this. Now what they are doing is most definitely not legal, and if they were to let you go during your probation then you would have a case and I'm pretty sure you'd win. But it's still a lot of time, effort and worry for you.

Personally, I think that you need to decide whether or not you this job with the "new" company. If you do, then I would say to them that you will take the job, but that you need all your benefits - length of service etc - transferred over and you are not agreeing to 3 months probation. Remind them that by law you have to be offered a position with your original company that is the same level or higher than when you went on maternity leave, and you are accepting that they aren't doing this, but that there is no way you are going through probation again. They will know full well that they can't do this, so hopefully will accept it without too much arguing. If they do arc up, give them the details of who you spoke to about your rights and I think that will quieten them pretty quickly.

I am sorry that you are going through this. It really sucks that they are not doing the right thing by you. I am sure it will all work out for you though. Best of luck.