  We present constraints on primordial B modes from large angular scale cosmic
microwave background polarisation anisotropies measured with the Planck
satellite. To remove Galactic polarised foregrounds, we use a Bayesian
parametric component separation method, modelling synchrotron radiation as a
power law and thermal dust emission as a modified blackbody. This method
propagates uncertainties from the foreground cleaning into the noise covariance
matrices of the maps. We construct two likelihoods: (i) a semi-analytical
cross-spectrum-based likelihood-approximation scheme (momento) and (ii) an
exact polarisation-only pixel-based likelihood (pixlike). Since momento is
based on cross-spectra it is statistically less powerful than pixlike, but is
less sensitive to systematic errors correlated across frequencies. Both
likelihoods give a tensor-to-scalar ratio, r, that is consistent with zero from
low multipole (2 <= ell < 30) Planck polarisation data. From full-mission maps
we obtain r_0.05<0.274, at 95 per cent confidence, at a pivot scale of k = 0.05
Mpc^-1, using pixlike. momento gives a qualitatively similar but weaker 95 per
cent confidence limit of r_0.05<0.408.
