Thanks everyone for your thoughts and comments, both for and against. That is why I asked a group of total strangers after all! To be clear, I very much would like another baby, for me personally, and for dd to have a sibling, and for new baby to have sibling. For us to be a bigger family. I thought if I put that on here, I'd get slated for being selfish. I want another baby, and I want the sibling element.
I do hold the opinion that having siblings is in general a good thing (learning to share, company etc) even if not best friends, a sibling is still for most people someone who will be there for you no matter what. Obviously there are plenty worse things in this world, but I would like this for my dd. not everyone will agree with me if course, and that's fine.
I do have support from my parents and other family with children nearby, my mum was an only child and she is one of the people who has influenced me on this-she hated being an only child, and I know that my parents would continue to help if this did happened.
I know it would be very hard especially for the first few years, but after that I think that two children are likely to play with each other to some extent which will make parenting easier. I'm not saying easy at all, as I'm sure lots of you can agree.
I only split up with ex recently and we have a decent relationship. He babysits and has dd for 1/2 days at weekends in general. We don't have a formal money arrangement, and to be honest I wouldn't imagine he'd contribute more, but he is a loving dad.
I honestly am not trying to snare him back. If anything having dd was what ended our relationship, so having a second would absolutely not be the way to do it, if that's what I wanted! I would be much more nervous of having a second child if we were together. He wasn't much help and we argued over lots of things like breast feeding and sleep (the usual).
No judgements about kids by different dads, and perhaps that will happen for me one day. I just can't imagine bringing another man into our lives at the moment and would want to be sure before I had a child with someone else. If that did happen there would necessarily be a bigger gap which in my opinion makes good sibling relationships less likely (though not impossible at all), and the different lives but comes in because I meant that ex would obviously still have time with dd, but not a half sibling-therefore creating 'different lives'. And dd would feel different to any half siblings who didn't go for time with my ex.
I do appreciate all your comments, it has made me think, which was the point. I don't honestly know what my ex would say.....