The Demon Club (Ben Hope 22) by Scott Mariani

Ben is flying home to Le Val in Normandy after visiting his girlfriend Grace in Scotland. A man sits next to him and persuades Ben to assassinate a former SAS colleague. Ben's motivation? If he doesn't do it, then not only is Grace's life in jeopardy, but also his sister, Ruth's life. It's a no brainer.
Ben is to tell nobody what he's doing, but his friends Jeff and Tuesday guess he needs them, so they follow and help Ben in his mission.
Ben tracks his target down to Spain, where they join forces and turn the tables on the men who employed them both for separate missions.
The quest takes them to the Isle of Man, Scotland and a country house in southern England. The men they're after are in a secret society that worships satan, called the Pandemonium Club. These men are judges, police officers, MPs and all sorts of high profile jobs, who believe they are above the law. They soon realise their mistake, as Ben and his colleagues track them down and dispense their own form of justic