javelin event at the English Schools' Athletics Championships. She was selected to compete in the javelin throw at the 1973 European Athletics Junior Championships the following year, where she reached the final but finished 12th with a throw of well behind the winner, Tonya Khristova of Bulgaria, who threw . Sanderson then decided to focus on the javelin throw rather than the pentathlon, partly because she thought that javelin competitions would provide more opportunities for travel. She made her senior international debut in the javelin throw at the 1974 British Commonwealth Games, finishing fifth. Later that year, Sanderson finished 13th in the 1974 European Athletics Championships. She broke the British javelin-throw junior record five times, achieving a distance of in 1974. Sanderson set the national record in 1976, throwing , and went on to set ten national records and five Commonwealth records. The 1976 season saw Sanderson's debut at the Olympics. Aged 20, she was the youngest competitor in her event and threw to finish ninth. In July 1977, at the European Cup semi-finals in Dublin, she threw a national record and the second-longest distance by a woman at the time. At the European Cup finals, Ruth Fuchs of East Germany won the gold and Sanderson took the silver. Later that year, Sanderson was the bronze medalist at the 1977 IAAF World Cup. Sanderson won her first major gold medal with a throw of in the 1978 Commonwealth Games, the first time England had won Commonwealth gold in the women's javelin since 1962. A few weeks later, Sanderson took silver at the 1978 European Athletics Championships behind Fuchs; she was the bronze medalist at the 1979 European Cup again behind Fuchs, both of them losing out to Romanian Éva Ráduly-Zörgő. Selected for the 1980 Summer Olympics, she failed to meet the qualifying standard for the final, reaching only with her first throw and having her other two attempts declared no-throws. After the 1980 Summer Olympic Games, Sanderson asked Wilf Paish of the Carnegie Institute of Physical Education in Leeds to become her coach, and lived with his family once he agreed. A throw of was enough for Sanderson to win at the 1981 Pacific Conference Games. At the 1981 European Cup, she was runner-up behind Antoaneta Todorova of Bulgaria who made a world-record throw of . She also competed in the pentathlon and heptathlon, setting UK and Commonwealth records for