Coherent optical-fibre sensing based on environmentally induced phase modulation can be extremely sensitive, but interferometric detection limits dynamic range. This letter examines a separate class of applications where the very high sensitivity is not required, but where remote optical sensing of relatively large physical variations is desirable. We describe a system whereby source modulation may be used to determine the state of a remote interferometer accurately and unambiguously over phase modulations from a fraction of an optical wavelength to tens or even hundreds of wavelengths. The concept may also be utilised as the basis of a multiplexed sensor highway, and preliminary results are reported.