I didn't start this until Thursday morning and spent most of the day wading through it. Once I'd filled the grid and seen two thematic words (one not in the two main dictionaries and not easy to confirm on the Internet either) I spent ages looking for what is needed for a safe ascent or descent. In the end I sneaked a look at this site for a hint, which I found at #2. This must be one of the most incongruous, poorly signposted finales ever, completely unthematic. The preamble is ambiguous regarding the highlighting (it could mean 25 plus), lots of the clues have inelegant wordplay that would not be allowed in the Listener for one, and some include otiose words. The supposedly & lit 28ac isn't truly & lit because of the word 'unfortunately', which means the defintion is inadequate.
The only positive thing I have to say about it is that the letters to be moved in the down clues were very neatly contrived and hard to detect.