  Precise control of the resonant frequency of a spin qubit is of fundamental
importance to quantum sensing protocols. We demonstrate a control technique on
a single nitrogen-vacancy (NV) centre in diamond where the applied magnetic
field is modified by fine-tuning a permanent magnet's magnetisation via
temperature control. Through this control mechanism, nanoscale cross-relaxation
spectroscopy of both electron and nuclear spins in the vicinity of the NV
centre are performed. We then show that through maintaining the magnet at a
constant temperature an order of magnitude improvement in the stability of the
NV qubit frequency can be achieved. This improved stability is tested in the
polarisation of a small ensemble of nearby $^{13}$C spins via resonant
cross-relaxation and the lifetime of this polarisation explored. The
effectiveness and relative simplicity of this technique may find use in the
realisation of portable spectroscopy and/or hyperpolarisation systems.
