A simple, self-diagnostic strain sensor is described, based on a strongly reflective optical fibre Bragg grating illuminated by a broadband source. The total reflected power from these gratings is shown to be a function of the strain gradient experienced by the grating. This is because a change in pitch within a section of the grating results in the emergence of reflected energy in other spectral regions, without any significant reduction in the peak intensity at the Bragg wavelength. Thus, the presence of a localized strain can be inferred directly from an intensity measurement without the need for an optical filter or other more complex interrogation schemes. For spectrally flat light sources, the measurement is relatively insensitive to environmental temperature changes. The sensing mechanism can also be considered 'self-diagnostic' as a signal is returned by the grating even under zero load unless the sensor has failed. Modelling results are presented to determine the minimum grating strength required to achieve this effect, while the technique has been experimentally verified by measuring the strain transfer on a loaded scarf repair joint at room and elevated temperatures. The scarf repair was loaded to failure and a reduction in strain transfer was observed as the failure grew along the bondline, in accordance with finite element modelling results.