The Judas Goat by Robert B. Parker

Spenser has gone to London -- and not to see the Queen. He's gone to track down a bunch of bombers who've blown away his client's wife and kids. His job is to catch them. Or kill them. His client isn't choosy. But there are nine killers to one Spenser -- long odds. Hawk helps balance the equation. The rest depends on a wild plan. Spenser will get one of the terrorists to play Judas Goat -- to lead him to others. Trouble is, he hasn't counted on her being very blond, very beautiful and very dangerous.