Nitrogen-vacancy centres (NVs) in diamond are attractive for research straddling quantum information science and nanoscale magnetometry and thermometry. While ultrapure bulk diamond NVs sustain the longest spin coherence times among optically accessible spins, nanodiamond NVs display persistently poor spin coherence. Here we introduce high-purity nanodiamonds accommodating record-long NV coherence times, >60 us, observed via universal dynamical decoupling. We show that the main contribution to decoherence comes from nearby nitrogen impurities rather than surface states. We protect the NV spin free precession, essential to DC magnetometry, by driving solely these impurities into the motional narrowing regime. This extends the NV free induction decay time from 440 ns, longer than that in type Ib bulk diamond, to 1.27 us, which is comparable to that in type IIa (impurity-free) diamond. These properties allow the simultaneous exploitation of both high sensitivity and nanometre resolution in diamond-based emergent quantum technologies. ; Comment: Additional data and analysis in PDF format is available for download at the publications section of http://www.amop.phy.cam.ac.uk/amop-ma