Upper panel: offset between the radial velocity of the PNe in common between the Hui et al. (1995) and the FLAMES observations. The mean offset (Hui – FLAMES) and standard error is −3.7 ± 31.7 km s^-1 (291 PN in common). The error bars are the quadratic sum of the FLAMES and Hui errors. Lower panel: offset between the radial velocity of the PNe in common between the Peng et al. (2004c) observations and the FLAMES spectra is shown. The mean offset (Peng – FLAMES) and standard error is −9.9 ± 41.5 km s^-1 for 645 spectra in common between Peng, SV and P73 FLAMES programmes. After removing 36 matches with the most deviant offsets (| Δ | > 50 km s^-1), the mean offset (Peng – FLAMES) and in particular its standard deviation are significantly lower: −7.9 ± 15.5 km s^-1. The error bars are the quadratic sum of the FLAMES errors and an assumed 17.0 km s^-1 error for measurements given in Table 6 of Peng et al. (2004c).