Thank you TwoDots.
I think the main reason why we 'hated' each other was because we made wrong assumptions about each other and because I think my ex was a bit selective about what he was telling her.
Understandably, he was trying hard to make a good impression, so he wasn't going to say to her that if he didn't have our children overnight, it was because he wanted to be able to go out Saturday nights and sleep in in the mornings. Similarly, he wasn't going to explain to her that the reason why I kept the house (bought him out) was because he had accumulated massive debts that I knew nothing about until they took him to court and put a charge on the house. So with being selective about what he told her, she built this picture of me being overprotective of the children and making it difficult for him to have access as well as someone manipulative that had gone out of my way to rip him off.
On my side, I made the error to judge her as a single mum on benefits, too lazy to work, playing the system and too much into her physical appearance. I also got on the defensive as she encouraged my ex to start making demands on him as a father, including having them for Christmas to spend time with her family when until then, we'd managed to be civil and spend it together so that we could both enjoy the day with them.
That's only a summary of it, there was of course a lot of instances that made us feel the way we did.
What made things turn around is that I realised that she did care about my kids, treated them fairly and respectfully, however never tried to act as a mum to them. By that I mean that, in my view, she had the perfect balance, for instance, she would ask them about their forthcoming exams, talked about how they were preparing, giving them her eldest books etc... but she never demanded to see their school reports or asked to go to parents' evenings.
Similarly, she realised that I was the money grabbing ex that she first imagined as I never asked for a penny maintenance, even when my ex, and then her started working. Inevitably, some truths about my ex's past also started to come to light and she realised that he wasn't all he had claimed to be, which even led them to break up at some point.
On the other hand, I have raised them to be respectful of everyone, and obey orders that come from any adults. If she asked them to clear up after themselves, they knew they just had to do as told and I would have told them that myself if they'd moan to me (they never did).
I really do think that the best way to move forward is to swallow a lot of resentment and frustration, let things go and trust that time will make things better as we accept things. Like most mums, exes, SM, there have been times when I felt like screaming that things were unfair.
Of course I still at times feel some resentment, especially when my OH has to do his tax return because although he earns under £50K, he has a company car and private insurance which mean that he is over the threshold for child benefit and needs to pay tax on it (but under £60K which would mean I would just stop claiming), yet their father doesn't pay a penny for them, not even their bus fare for them to go and see him every week-end. I have decided to let it go though because in the end, it is his choice and if he thinks that is ok, then I can't change that.