The effective demultiplexing of WDM signals requires a tuneable filter, which is able to arbitrarily select and filter more than one channel to a particular output at any one time. Thus, such optical filters need to posses a large dynamic tuning range, narrow bandwidth and high sidelobe-suppression capabilities. Important applications of these tuneable filters include Add/Drop multiplexers and wavelength-selectors for tuneable lasers. The proposed architecture consists of cascading a Polarisation Independent Acousto-Optic Filter (PIAOTF) with a Fabry Perot Interferometer (FPI). The frequency domain characteristics of this architecture outperforms those currently offered by tuneable technologies, including the Fibre-Bragg Grating, the Mach-Zehnder Interferometer, the Fabry-Perot Filter and the Double-Stage PIAOTF. Consequently, our architecture is very favourable for WDM purposes since the filter boasts a sidelobe-level attenuation of <-25dB when simultaneously switching four channels, whereas the leakage at the through-port is only -16dB. In addition, the filter proves to demonstrate a convincingly flat passband with a 3dB bandwidth of ~0.35 nm. Consequently, a simple model is built to simulate the WDM (demultiplexing) environment in which the novel tuneable filter may be deployed. In this case, the tuneable filter's architecture enables a light stream consisting of multiple wavelength-channels, each with a spectral width of 0.1nm, to be split into two output ports via its bandpass and notch spectra. Hence, there is both a filtered and non-filtered light stream.