Dare Call It Treason — Bob Hyslop

1956 UK. Stalinists plot to murder the new Soviet leaders, Bulganin and Khrushchev, visiting Britain. But their agent a themselves as much as their enemies.
Jonas Forbes, hired to protect the visiting leaders, soon finds himself with a personal score to settle. He is both helped and hindered by the rival Soviet organisations, the GRU and the KGB, as well as the British police.
A lightly-written thriller concentrating on historical accuracy.