It is been a familiar story in recent years: a Budget that sounded reasonably good when delivered, but that unravels in subsequent days. Rishi Sunak’s spring statement was no exception. When he delivered it a fortnight ago, he said he was going to compensate low-earners by raising the primary threshold for National Insurance, bringing it into line with income tax and relieving people who earn less than £12,500 from having to pay NI at all.