To work out the equivalent return on a taxable account, gross up your mortgage rate by your marginal tax rate.

To achieve this with a taxable account, basic-rate taxpayers must find interest of 0.38 per cent a year, while higher-rate taxpayers need 0.5 per cent.

Basic-rate taxpayers would need to earn about 1.9% in a taxable account to earn 1.5% net, while higher-rate taxpayers would need to earn 2.5% and top-rate taxpayers 2.75%.

In order to get a 9% rate of return in an ordinary taxable account, in a 34% tax bracket, one must earn 13.64%.

This equals the dollars of tax that would have been paid on the income if earned in a taxable account.