Chrisremo, as someone with 20 years’ experience of the Listener, I’d suggest that this is very much at the tougher end of the spectrum, particularly nowadays. Frankly, I don’t think a novice would be able to finish without being spoon fed half the answers.
I had to cold solve almost half the clues before seeing a way to enter some, but I enjoy doing it and probably did more than necessary. Generally I’d have thought people will need to have solved at least a third. Clearly the top line will help, and explanations of the first two clues have already been given.
It’s certainly worth playing with Mooncow’s lovely tool mentioned early in the thread to give an idea of how the grid ought to look. Don’t worry too much about the extra letters, it’s the down clues’ positions which can be fixed.