I picked this up because it mentioned Gaping Ghyll and because the author also has a Boudicca set, both of which are interests, and general reviews suggested a good thriller. The author's writing style is ok, good at times, but for all the claims for research - the results are patchy at best.
If you're a caver, know the Dales and/or Gaping Ghyll - it will either be annoying or pretty amusing. Limestone and chalk cave? Don't think so. A handful of caving/equipment words thrown around without much understanding? Cave rescue (or others who cave) not ever having been to the base of the pitch/cavern(sounded a bit like Mud Hall)? Dry suits?! Man falls 400 feet and survives...same man then lies in water for hours and survives...maybe if he was floated down by the power of the skull and then protected from hypothermia. And why do some authors insist on featuring main characters with skills they themselves know nothing about? But hey, I was willing to ignore some of it, though the repeated bits of 1/2 knowledge and rubbish don't inspire confidence in the research areas I don't know anything about. And just human nature - you hear someone else in a cave with you - you automatically assume they have evil intent? I think most people would wait and see who it was rather than sending the guy who's only been in a cave once running off.
The characters were only so-so and motivations vague. The major danger of shared time/character stories happened and I started skipping bits to read one story more, than going back. Then I made the mistake of reading the author's notes and lost all interest. I gave it up.
Have to agree with other criticisms in the reviews - if it was Dan-Browning - the basic writing is probably better, but he gets away with it, presumably, because he throws so many ideas, mysteries,etc at you. And given this, I'm not sure I'll bother checking out the Boudicca. Disappointing.