Here we present the design and performance of a novel unilateral finline Nb-AlOx-Nb SIS (Superconductor-Insulator-Superconductor) mixer, operating around 230 GHz, with a target IF bandwidth of 2-13 GHz. The mixer is intended to be used in GUBBINS , a prototype high brightness sensitivity, low spatial resolution heterodyne interferometer. A key feature of the mixer design is the use of a unilateral finline taper to provide a smooth transition from high-impedance waveguide to low-impedance microstrip suitable for feeding a 1μm2 SIS junction. The use of a finline transmission line allows other complicated planar circuits to be compactly integrated on the substrate and allows the use of an easy-to-fabricate mixer block. Also the employment of the silicon substrate allows trenches to be fabricated around individual SIS mixer devices on the wafer, allowing the devices to be separated easily without dicing. To realise the wide IF bandwidth, a separate IF matching board, consisting of a few sections of microstrip, was designed to match the dynamic output impedance of SIS mixer to the LNA. In this paper, the SIS mixer design will be described in detail, including the electromagnetic simulations of the passive circuit with HFSS. We have fabricated and tested several of these SIS mixers over RF bandwidth 190-260 GHz. We have obtained a best noise temperature of 75 K at 208.8 GHz over IF bandwidth 4-6 GHz. However, the noise temperature measured across IF bandwidth 2-18 GHz shows that an IF resonance exists around 8 GHz, caused by an excess capacitance due to the large surface area of the microstrip transition and RF matching circuitry. An improved design is described, suitably modified to shift the IF resonance out of the target IF band.