chancellor faces calls to rule out further tax cuts for the uk 's top earners .
he dodged questions over plans to offer more help to £ 50,000 plus salaries .
instead he focused on raising the amount of tax-free income to £ 12,500 .
labour plans to impose 50 per cent tax rate on salaries of £ 150,000 or more .
the current rate for the top tax bracket is 45p with basic standing at 20p .