spabby full time it’s £4,600 pa for the 2022-23 academic year, half that for part-time. That’s for history at both York and the OU, so I imagine others are similar.
The process isn’t any different for older people - think about the topics/themes you want to research; think where you want to study, bearing in mind that distance learning is now common; and then check if those places have potential supervisors - all their research interests will be on the Uni’s website. It’s worth discussing your ideas with those who seem a good fit before making a formal application.
All the university’s have ‘how to guides’.
I started a DPhil after my first degree, in my mid-30s, but didn’t complete it. I’ve done two Masters degrees since then, one overseas, and am now going through the process of deciding where, and with whom, to start a new PhD next year.