In this novel, a girl goes to (northern England? southern England? Cornwall? The UK somewhere). I think she's looking for her father, who was/is an archaeologist. The dig is looking for (Roman? Bronze?) artifacts. Ultimately, there's a (bronze?) helmet that the villain is willing to kill for. One of the villains is the much-younger wife of an archaeologist who is described as a nymphomaniac. Towards the end of the book, the heroine and the love interest are, I think, trapped underground. They locate the helmet and I think a corpse down there? Ultimately, if I recall correctly, the nymphomaniac villain is washed away and drowns.
The book was a library edition, hardcover, and I think it was green but could be wrong. It was set in modern times, and since it was old when I checked it out in the early 2000s, was probably written late 70s-late 90s. I read it in English, when I was somewhere between thirteen and fifteen, and it was a bit age-inappropriate (what with the nymphomaniac, lol - that was the first time I ran across that word, which is probably why it stuck with me). It was clearly intended for adults. The author's name was probably somewhere in the middle of the alphabet, since I think I found it when I was looking for Barbara Michaels novels.
This has been bothering me for about fifteen years now. I would love to reread it - I don't even care if it's good, as long as I FINALLY know what the darn book was.