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 σ(r) of the order of ≤ 10-2. 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.