Neil MacGillivray: Is BoJo's tax promise good news or a 'red bus'?

When Boris Johnson announced that, if he becomes the next prime minister he would increase the 40% tax threshold from £50,000 to £80,000, there was a degree of scepticism as to whether he could deliver on what at first appears to be an exceptionally generous promise. Might this just be another exaggerated claim similar to the infamous advert that exiting the EU would lead to another £350m a week going to the NHS? There have been no red buses in sight just yet but, then again, little in the way of detail has been revealed - other than the cost to the Treasury being in the region of £9.6bn...