Thank you for all the replies, comments and suggestions. I wanted to clarify a few things. A few people have said that I am judging the Mums parenting and being harsh on her and focusing on her. There are so many layers to this situation and obviously I cannot list everything, I also appreciate that people can only comment based on what they read on this thread.
The Mum's parenting and behaviour does come into the equation when dealing with this situation with my SD because a lot of the issues we have surround the way she parents at home and makes comments like 'you can do that at my house but I know Daddy doesn't let you do that at his house' - she should 100% be allowed to parent her own way in her own house, but it is affecting our house as her wording to us and my SD make it feel like she is the fun house and our house has rules and boundaries. However, minimal they are compared to hers.
One example is a Friday night sleepover - on the Fridays my SD is with her Mum, she tells her partner he has to sleep on a makeshift bed on the floor or on the sofa as they are having a sleepover and he must therefore leave the bed for that night. She has stated to my partner that he should be doing the same, that the sleepovers are important to his daughter and I should therefore leave my bed and sleep elsewhere on those nights. My partner does not agree with this and so it does not happen. I should also point out that I have back issues here and there so sleeping on a sofa is not appropriate. I have now suggested he put an airbed up in her room on those Fridays so they get a sleepover but I am not forced out of my own bed.
There have been many occasions whereby she has phoned my partner to tell him a complaint about me and I have been in the room and heard the call. Sometimes, when I have seen her and discussions have been had, I have asked her about things that were said on the call, but told a lie and said it was re-laid to me rather than I heard it - she has always denied what was said or given a different version of it. It is things like this that make us question what we are being told about my SD's issues with me and our house.
Another example of this is we were at a Sports Day event and me and her were talking. She told me that her partner wants to move away from the area as it is so expensive to live and she agrees that at the end of Primary School it would be a good move as she could buy somewhere bigger and be nearer to her Mum. I said my partner wouldn't like that as would mean even more limited time with his daughter. She said that my partner was narrow minded and needed to look at the bigger picture. I said I agreed with how expensive the area was and that we had recently visited the Cotswolds and I would love to move there. A pipe dream, nothing more. Fast forward a few months and during a phone call to my partner she said she knew I was trying to get him to move away with me..... no mention of her move or the context of my comment at all.
2 nights ago was when the recent things came to a head for the 2nd time. She phoned my partner and told him how much his daughter disliked me and didn't like being in the house with me, that I wasn't friendly and I had caused a huge rift in the dynamics - I heard the whole phone call which lasted 30 minutes of me being slated. The next day I promised my partner that I would do all I could to fix things as it is crushing him. I sent her a text saying how devasted I was to learn of my SD's feelings towards me and how could we work on fixing it. She replied and said that she had not said those things to my partner, that he had gotten things wrong when telling me. She said there is no huge issue, that my SD is just emotional right now and needs her Mum more then us.
There have been lots of comments about me being insecure and changing their relationship as a family. That is simply not true. When I met my partner, they had been divorced about 6 months and in their new homes for 5 months. That was when they were doing days out, lunches, teas etc, they were finding their feet. I never once asked him to stop. She met her partner at a similar time to us meeting. Over time, we noticed a pattern of behaviour - she would ask to meet him and their daughter for lunch, but only when her own partner was busy or away with work. She would invite him in for tea, but only if her own partner was not there. The days out would only ever occur on the weekends she had her daughter, when he suggested a day that he had his daughter, she would say she couldn't do that weekend. That is why me and him had discussions about why these events were taking place and the reasoning behind it. Was it really what their daughter needed or was it because she did not like being on her own with her daughter. It seemed that she didn't wish to meet up with my partner and their child when she had her partner around, it was only when she was on her own. One weekend, she asked to meet my partner with their daughter for breakfast, then lunch, then dinner - we found out her partner was away that weekend.
And no, before anyone suggests it, it is not because her partner wouldn't like them meeting, he is chilled with that side of things.
Yes, they used to get on very well and that has changed. Not because of me like many of you have said. It is because she started lying and manipulating my partner, if he said no to something, she would get their daughter to ask him instead. If she wanted to plan a day out, she would plan it with her daughter before even asking him, then get their daughter to call him and say how much she really wanted to go to XXX. She took her out of school for a day and didn't consult him, when he said she should of, she said I am DD primary carer and I can make these decisions without you. There are many many many more examples.
Yes, I encouraged him to stand up for himself on these occasions, is that so wrong. Their relationship changed because her behaviour changed. I should not be blamed or pegged as the jealous new partner, that is so unfair.
These are a few examples of things that have taken place and why we do question her version of things and her motives. It is hard not to after everything that has happened.