  Future Cosmic Microwave Background (CMB) satellite missions aim at using the
B-mode polarisation signal to measure the tensor-to-scalar ratio $r$ with a
sensitivity $\sigma(r)$ of the order of $\leq 10^{-3}$. Small uncertainties in
the characterisation of instrument properties such as the spectral filters can
lead to a leakage of the intensity signal to polarisation and can possibly bias
any measurement of a primordial signal. In this paper we discuss methods for
avoiding and correcting for the intensity to polarisation leakage due to
bandpass mismatch among detector sets. We develop a template fitting map-maker
to obtain an unbiased estimate of the leakage signal and subtract it out of the
total signal. Using simulations we show how such a method can reduce the bias
on the observed B-mode signal by up to $3$ orders of magnitude in power.
