There is a heat wave in London and the sporting calendar for the summer is full and exciting. However, there could be an attempt to ''fix'' a major horserace; political demonstrations are likely at a cricket match; and obstacles are put in the way of a young African-American succeeding at Wimbledon. Plenty of headaches for Commander George Gideon of Scotland Yard - and plenty of crime - not to mention trouble at home. Will he cope?