SIR Alastair Morton, the chairman of the Channel Tunnel operator Eurotunnel, is to step down after he has concluded key negotiations with its banks over the company's £9 billion sterling debt mountain, it was announced yesterday. The former merchant banker became chairman of the British half of the Anglo French group in February 1987 and is also co chairman of the joint holding company. He was also chief executive from January 1990 to January 1994, when he often clashed with the consortium of UK and French companies that built the tunnel and governments on both sides of the English Channel.