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We demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy colour centre in diamond at cryogenic temperature. An open cavity fabricated using focused ion beam milling provides mode volumes as small as 1.24 μ m ^3 (4.7 ${\lambda }^{3}$ ) and quality factor $Q\simeq 3000.\;$ In situ tuning of the cavity resonance is achieved with piezoelectric actuators. At optimal coupling to a TEM _00 cavity mode, the signal from individual zero phonon line transitions is enhanced by a factor of 6.25 and the overall emission rate of the NV ^− centre is increased by 40% compared with that measured from the same centre in the absence of cavity field confinement. This result represents a step forward in the realisation of efficient spin-photon interfaces and scalable quantum computing using optically addressable solid state spin qubits.