By Jack Barnett House prices could tumble as much as 21 per cent if the Bank of England is forced to hike interest rates steeply in response to the government’s tax and borrowing splurge fuelling inflation, City economists have warned. Mortgage affordability – the ratio of a home purchase loan to the size of the borrower’s income – may have to drop from 3.5 to 2.5 if lenders start charging six per cent on mortgages. “If that happened overnight, it would imply a 21 per cent fall in house prices,” according to Andrew Wishart, senior property economist at consultancy Capital Economics. Higher rat…