We describe a novel heterodyne interferometer currently under construction at Oxford. The instrument employs new techniques in heterodyne interferometry, with the aim of achieving very high brightness sensitivity in the millimetre band. It is a single-baseline tracking interferometer for operation in the frequency range 185-275 GHz with two 0.4m offset parabolic antennas separated by a 0.5 m baseline. Each antenna feeds an SIS mixer with a 2-20 GHz IF band, driven by a phase-switched LO source. The IF signals from the mixers are processed by a 2-20 GHz analogue complex correlator. The primary science goal of this instrument is to measure the spectrum of the Sunyaev-Zel'dovich effect in galaxy clusters. In particular we intend to measure the frequency of the S-Z null near 217 GHz, which allows the gas temperature of the cluster to be determined. Measuring the spectrum of the S-Z effect requires very high brightness sensitivity with moderate spatial and spectral resolution. I. INTRODUCTION Cosmic microwave background astronomy requires extremely high brightness sensitivity and very good control of systematic and instrumental effects. The use of heterodyne interferometry techniques allows a number of instrumental effects to be removed, and also removes the instrumental sensitivity to total power fluctuations, suppressing the effect of atmospheric noise fluctuations. The use of heterodyne receivers also allows each receiver to be phase switched individually with the local oscillator, so that individually modulated redundant baselines can be used to eliminate instrumental effects. Achieving high brightness sensitivity in interferometry requires that the instantaneous bandwidth of each baseline be as wide as possible, and that the array be as filled as is practical within the limits of antenna shadowing. Interferometry has been widely used in cosmology instruments at centimetre wavelengths, particularly for observations of the primary temperature anisotropy and E- mode polarisation of the cosmic microwave background e.g. CBI, DASI, VSA, and in observations of secondary anisotropies such as the Sunyaev-Zel'dovich effect, e.g. AMI, CBI-2, SZA. Although measurements of the CMB and S-Z effect at millimetre wavelengths are extremely useful, the limitations of the low instantaneous (IF) bandwidth of SIS mixers and backend systems, and the poor noise performance of other mm-wave coherent detectors are responsible for the absence of successful CMB instruments using heterodyne interferometry in the high millimetre-wave band. Recent advances in SIS mixer design and wideband correlator technology make it feasible to build a mm-wave heterodyne interferometer capable of carrying out novel CMB observations.