Nicola Sturgeon is on course for an SNP majority of one in the Holyrood election opening the door to a constitutional showdown with Boris Johnson and fresh demands for a second referendum.The poll found support for independence is still on a knife edge with 47 per cent against separating from the UK with 46 per cent in support.Boris Johnson is expected to tell the Scottish Tory conference on Sunday that he will refuse permission for a second referendum even if the SNP wins a majority in May.John