  The observation of the polarised emission from the Cosmic Microwave
Background (CMB) from future ground-based and satellite-borne experiments holds
the promise of indirectly detecting the elusive signal from primordial tensor
fluctuations in the form of large-scale $B$-mode polarisation. Doing so,
however, requires an accurate and robust separation of the signal from
polarised Galactic foregrounds. We present a component separation method for
multi-frequency CMB observations that combines some of the advantages of
map-based and power-spectrum-based techniques, and which is direcly applicable
to data in the presence of realistic foregrounds and instrumental noise. We
demonstrate that the method is able to reduce the contamination from Galactic
foregrounds below an equivalent tensor-to-scalar ratio $r_{\rm
FG}\lesssim5\times10^{-4}$, as required for next-generation observatories, for
a wide range of foreground models with varying degrees of complexity. This bias
reduction is associated with a mild $\sim20-30\%$ increase in the final
statistical uncertainties, and holds for large sky areas, and for experiments
targeting both the reionisation and recombination bumps in the $B$-mode power
spectrum.
