The resonance wavelengths of Bragg gratings are shown to increase with irradiation time during their inscription if the core of a germanosilicate fibre is exposed to a 243 nm UV fringe pattern. Exposure of the gratings to a 243 nm UV monotonic illumination leads to a recovery in the transmittance of the filters. Whether this irradiation induces a shift, or not, in the Bragg wavelength of the filters depends on the history of the grating writing experiment.