A novel singlemode optical fibre surface plasma wave sensor which operates over an extremely broad spectral range is reported. This is achieved by coating a gradually tapering singlemode fibre with a thin layer of silver ~50 nm thick on top of an adhesive layer of chromium 3 nm thick. The gradual change in the taper cross-section results in distributed phase matching between the fibre mode and the surface plasma wave, permitting a plasma wave to be excited over a spectral range from ~600 to 1000 nm. The device shows extreme sensitivity to small changes in the external refractive index, with changes in refractive index of ~5 × 10-4 being detectable. The device has considerable potential as a biochemical sensor using surface immobilisation techniques.