Has the government been overestimating net non-EU immigration?

Could next Monday present another moment of embarrassment for the government on immigration? As figures are finally due out on net migration - delayed from May - the signs are somewhat ominous, writes Marely Morris (IPPR). He argues that it's looking increasingly possible that the government's central estimate of migration - based on its longstanding International Passenger Survey - could be overestimating net flows from outside the EU.