Not much more to say, there's a website called Fantastic Fiction where you can look up all of an author's books. But the book I have is called Escape to Darling Cove, with the same cover as "Hidden Depths", all 4 mini novels have now been put together in the one book.
They've all been published in one book now, I think the first cover in your post, and the title, is the one they've used, I have it myself and also saw it in my local Sainsbury's.
that's one of my favourite books in the series, I found Stranger from the Sea highly disappointing (all that stuff about steam, zzzzz) and was worried the series had lost it's way a bit, but this one got it right back to how it was before "Stranger".
This was one of my favourites in the series, partly because a lot of it is set in and around the area I used to work in, so I could imagine the settings very well.