John Powers is the managing director of ACC Trading Ltd, which provides components to the...

John Powers is the managing director of ACC Trading Ltd, which provides components to the automotive industry. His company has been providing number plates to Elite Motors Ltd for many years. As John Powers and Peter Ellis, the MD of Elite, have been friends for a long time, there has never been a formal contract agreed between the companies. Following a downturn in the market, Elite Motors Ltd has now been placed in administration. ACC recently delivered 200 number plates but have not received payment. As the number plates have been fitted to some vehicles, the administrators are being very slow to return them. Which clause in a formal agreement would have helped John?