In the novel They Came to Baghdad, Dr Smallbrook is an operative of of "The New Order" secret organisation who appears towards the end of the story.
The organisation has succeeded in kidnapping Grete Harden (whom they think is Anna Scheele). Victoria Jones (who has offered to help the "New Order") is sent in to impersonate her and she arrives and is checked into the Babylonian Palace hotel in Baghdad. From there, Dr Smallbrook calls the U.S. embassy. He says he is acting for Anna Scheele and has some documents which she wants them to pick up. Anna cannot come to the phone herself as she is ill in bed with laryngitis. The purpose of this ploy was to forestall the efforts of Anna Scheele. Anna Scheele was carrying important documents to Baghdad which threatened the New Order. The New Order wanted to plant fake documents into the hands of US embassy officials and lead them to think those were what Anna Scheele was bringing.
Later, Dr Smallbrook is arrested when Victoria is rescued by British intelligence agents. It is not clear if Dr Smallbrook is a real doctor. But considering that the organisation would expect that the people he is dealing with (such as the U.S. embassy) would check on his bona fides, they would be careful to use a real doctor or at least someone whose cover can withstand scrutiny.