The Chapter of St Cloud by Marcus Attwater

"Detective Inspector Collins stared at the whiteboard in his office. He hated the thing. It had photos of the murder victim stuck to it with little magnets, and his name in a big red circle in the centre. Other people had pictures of their family in the office. He had a boy in a blood-soaked T-shirt."

Dominic Walsingham is researching the Chapter of St Cloud, a religious order his fellow historians have mostly left alone until now. That's probably because the order still exists, and its members don't take kindly to inquisitive medievalists. Dominic is beginning to think maybe he should leave well alone.
Meanwhile, his colleague Claire is spending her holidays with her new in-laws. There are more of them than she can easily keep track of, and although they are very friendly, Claire doesn't really feel at ease. What are they hiding from her?
Both decide they had better take their suspicions to the police, but between investigating a series of drug-related murders and keeping his handsome young informant out of trouble, DI Owen Collins really doesn't have time for academic speculation…