Where do i stand?

I got divorced 17 yrs ago and a settlement was agreed of £9000 to be paid when my children left education at 18. It was all sorted through solicitors and seemed a lifetime away however my youngest with my ex wife will be 18 and leaving college to join the rat race in July but here is my problem. My ex wife has been the stereotypical bitter divorced wife and has made it incredibly difficult for me over the last 17 yrs, I have a great relationship with my 2 sons and speak to them nearly everyday, see them regularly and always have. I spent the last 3 yrs traveling 4 hrs each way to see my eldest while he was in uni and went to his graduation last year even though I wasn't invited to the family celebratory meal. Over the last 17 yrs I have moved several times and have lost all "papers" relating to the divorce. I have been to speak to the solicitor who i worked with but he doesn't recall any details and has told me they only keep correspondence for 7 yrs meaning my "case files" will be long gone. I don't even know who her solicitor was or if they're still in business so I can see her screwing me over. Any advice would be gratefully received (i have been to citizens Advice and they couldn't help me)