A method for fabricating special fibre, which offers considerable flexibility in both fibre structure and material, is described. The technique consists essentially of selecting borosilicate glasses from a wide range of available material, cutting and polishing rectangular blocks, arranging the blocks to provide any desired combination and configuration of guides and then heating and pulling down the stack to fibre dimensions. A number of possible fibre configurations are described, including couplers, polarization holding and the possibility of developing composite structures. One particular fibre structure, the single fibre Mach-Zehnder interferometer, is described in more detail, as an illustration of the potential of the technique. Some results are given of the operation of the interferometer as a sensor, using a laser source. Some of the difficulties of interpreting the output signal from this form of operation, and the advantages of incorporating a broad band source are considered. It is shown that a spectral analysis of the broad band output can provide an unambiguous interpretation of the state of the interferometer, and hence the measurand.