As far as I can tell, there's nothing actually wrong with Finding Camlann. It's well written, the characters are well-developed and believable, it's set in Oxford and it's (sort of) about King Arthur.
It follows Donald Gladstone, archaeologist, and Julia Llewellyn, employee of the Oxford English Dictionary, on a literary quest for the origins of Arthur in the hills of Wales. Sort of like The Historian but without the vampires.
I guess it just didn't grab me. It's a quiet, meditative sort of book, full of rain (well, it is Wales) and sadness, jilted lovers and broken hearts. And I found it hard to empathise with Julia because, well, she's married and carrying on with another man. But I realise that's quite an old-fashioned perspective, and someone else might enjoy it more. The rating reflects my personal engagement with the novel more than the quality of the writing.