Vagabond by Gerald Seymour

MI5 officer Winnie Monks has never forgotten - or forgiven - the death of a young agent on her team at the hands of a former Russian Army Major turned gangster. Now, ten years later, she learns that the Major is travelling to a villa on the popular Spanish holiday destination Costa del Sol, and she asks permission to send in a surveillance unit. They find an empty property near the Major's: the Villa Paraiso. It's perfect to spy from - and as a base for Winnie's darker, less official, plans. But it turns out the villa isn't deserted. The owners have invited a young British couple to house sit while they are away. Jonno and Posie, a new couple, think they are embarking on a romantic, carefree break in the sun. But when the Secret Service team arrives in paradise, everything changes.

Gerald Seymour is a perennial bestseller in the UK. He routinely receives rave reviews for his work, and The Outsiders is no exception--The Sunday Times (UK) called it "a sophisticated, reader-teasing tale," while The Sun (UK) praised it as "a gripping thriller."