3C31 is an FRI radio galaxy that has long been known (e.g. Burch 1977 ) to have a resolved bright jet and counter-jet with a strong initial brightness asymmetry, also present on parsec scales (Lara et al. 1997), a high degree of linear polarization at centimetre wavelengths (Fomalont et al.1980) and a significant depolarization asymmetry ( Burch 1979; Strom et al. 1983). It is identified with the dusty elliptical galaxy NGC383 (Martel et al.1999) that is the brightest member of a rich group of galaxies (Zwicky & Kowal 1968; Arp 1966) with an extensive hot intra-group medium (Komossa & Böhringer 1999).
We adopt a redshift of 0.0169 for the galaxy (the mean of values from Smith et al. 2000 , Huchra, Vogeley & Geller 1999 and De Vaucouleurs 1991 ), so the projected linear scale of the images is 0.34kpc/arcsec.