Thanks everyone for your time and your opinions. It's pretty shit, although very helpful for other people to confirm what I think I already knew. Just to answer some of the questions that have been asked.
Vimfuego No, the ex did not seem to put up any fight when he sent the email asking for visitation. He claims that in the past she hasn't responded but on this occasion, it was all agreed in 3 days. However, at the initial split she would not tell him her whereabouts or respond to contact.
Littlelions He lives about a 2.5 - 3hr drive (one way) from the children.
Osolea In terms of his relationship with the children prior to the marriage breakdown, he was SAHP for eldest child for about a year. When he went back to work he said that he did school drop offs and weekends in the park.
MsColouring Yes, I do wonder if much of his attitude relates to his inability to separate the children from the situation. I do understand how much a break up can hurt, I've been there, but I just think that as a parent you do not have the luxury of putting yourself first. I have suggested that he considers counselling but he said that he doesn't need it.
What I did not put in the original post as I could not decide if it was entirely relevant, or focus on ex's illness; was the fact that his ex had periods of severe mental ill health and had been hospitalised twice. It was after the third period of ill health where she went to stay with relatives and refused all contact and served divorce papers. But my view is that if you know that the main caregiver needs extra support, you have even more of a duty to ensure that you have pursued access.
Corlan Yes, he does financially support the children.
Onedirection No, I haven't been with him when he has been with the children, I have never met them. The resumption of contact has been relatively recent and I thought that he needs to re-establish a good relationship with them prior to them meeting me. I told him this from the outset. As to why I feel the need to have a child with him; well, that's possibly another issue clouding my thoughts. My age and the thought that I may not meet someone in time for me to have another child/ren. Whilst this is a real concern for me, it does not trump everything and I think I know deep down that this is not a relationship that I can progress with.
Daffodil Thank you for sharing your experience. No, he is not divorced yet. She started proceedings but they could not agree on the split of assets and they both appeared to just leave it. He is moving forward with it again as they have spoken and agreed on a way forwards with the properties.
My expectations about parenting are that your children are so precious and they need you even more when things get tough. My oldest is 18 and at uni, I speak to her everyday and tell her I love her. Her father, is one of the most emotionless people that I know. Him and his daughter have a great relationship and we split when she was 2 - he's just always made the effort.
His relationship with his family is poor (another red flag for me). His father does not live in the UK so less contact is understandable. He does not speak to his mother or siblings because he feels that they did not provide him with enough support when his wife left. They had contact with her and respected her wishes for them not to pass her whereabouts onto him. I can argue the rights and wrongs of this both way as the ex had been extremely ill and may have wanted to focus on her health or, it could have been a manifestation of her illness, I'll never know.
His parents also split up when his was a child and they split the kids between them. He lived with his father and saw his mum and siblings every year. The family dynamic appears odd to me as my mum would have gone postal on my brother or I if we didn't fight to see or children. His parents have seen the grandchildren once and, even more strange, didn't even attend his wedding saying that they were 'busy'. His family situation seems odd and I wonder if his hands off approach is also related to how he was raised in a seemingly very detached way.
The more I write I think the more I am answering my own question. It is very sobering to see it in black and white.