Keeping that letter was a very, very good move. Keep it safe (any chance he could burgle your house to get it?). Copy it and keep copies somewhere. Tell him you still have the letter he sent and that you will pursue him through the courts unless he makes reasonable provision for his child's future. Seek legal advice.
No apparently she said that he couldn't see his DS at all and he had no idea where they were. Suddenly out of the blue he got contact from the CSA. He was aware he had a son as your ex-p knew he had a child.
Perhaps one of the family lawyers could confirm this, but I think when your daughter turns 18 she can put in her own claim for maintenance, which would cover her until she finishes tertiary education. She could perhaps consider a a particularly long course, such as architecture or medicine...
Sorry, the CSA are only responsible for child support from the time they received your application and it's isn't possible to make a claim retrospectively for back payments before the date of the application.
The good news is that over 18s in education or training may claim maintenance in their own right from separated parents who can afford to contribute through the courts. Over 18s are eligible for legal aid for the time being at least.
As set out above, it can only be backdated to when the claim was made. What is it you think you can sue him for?
Your best course of action would be to push for the backdated maintenance from the date of application to the present and suggest to your DD that she makes an application in her own right for support through Uni.
A schedule 1 Children Act claim could be made I suppose for a capital sum to support her through university. The letter you've kept all these years might be useful. According to the NUS students without parental support will have debt of £60k at the end of a 3 year degree course.
But did you honestly think with another family & him never seeing her that he was going to put money aside for her? The CSA was set up to avoid the court process so I still wouldn't hold my breath, especially if he has another family to support. Good luck - he should pay towards his daughter!
My ex-husband deserted me and his 2 children in 1968 and I never heard from him again and I never received any maintenance from him what-so-ever. I remarried My 2nd husband Ed" in 1974 and we had to hire and pay for detective to find my ex' to ask his permission to adopt the 2 children! but he was never ever found. Ed" has been a great husband and father who treated and raised the children as his own." Yes' it happened a long time ago and i have remarried; my ex' died in 2010 leaving a considerable sum of money in trust to the women he lived with (they were not married). As I" then Ed & I" never received a penny in children support not even a birthday-card. Is it possible (legally) that I could sue for backdated children support? OR - Could I challenge the legality of the Trust fund? I don't want to go to a solicitor until I know if I have a chance of reclaiming these funds or not. Thanks for listening.!