That one thing is fairly important: the ending. Although I had a sense early on (correctly, as it turned out) of the who-dun-it of this mystery, I found the solution just lacking in believability. Murderers in mysteries need an adequate motive, and I thought that the motive was too little to explain murder. I can't say more without giving away too much.
But -- this is a complex and interesting puzzle mystery -- one of those mysteries where you solve it along with the police, as they go interview people, discover facts, interrogate suspects, etc. The plot involves a series of murders that take place in a small English city at a time when a well-known television journalist specializing in crime is in town working on a story. The killer appears to be engaged in a battle of wits with the journalist, suggesting that the motive is a grudge against the journalist, but who and why? The husband-and-wife police detectives have a small hand-picked group of staff working on solving the mystery before someone else gets killed. It appears that there's some connection to a string of gambling palaces (bingo, mostly) owned by a shady man who is living in a country house near the city.
This is perhaps the third mystery I've read by McGown, and I intend to go on and read more by her. She is not quite of the caliber of the great writers of English mysteries, such as Elizabeth George, P.D. James, or Deborah Crombie, but she writes quality mysteries that engage the mind as puzzles even as the reader enjoys the characters and following the background story line of the personal relationships of the detectives and their various problems (such as a live-in mother-in-law to care for their child).