Oh, so they told you how much they wanted to pay!! Interesting way of working.
If you are happy with the £25*, they have agreed to pay you, send them an invoice (if you haven't done already) add to this any correspondence you have, including telephone conversation details. Mark it for immediate payment (although I doubt they will pay it straight away, you'll have to keep chasing the payment). If it's not paid within say 30 days, then start proceedings to take them to the small claims court.
It's a right PITA chasing payment, and I don't know if it's worth all the effort for £25. So you might want to just send them a cease and desist notice if you want them to remove your image, or live with the fact that your image (and now your name) is being viewed by thousands of people visiting a national newspaper.
* Have a look at what AP4A has to say about calculating charges and the process he took, on the
link in my previous post.
Regards
Nick