  We present the highest resolution, wide-field radio survey of a nearby
face-on star-forming galaxy to date. The multi-phase centre technique is used
to survey the entire disk of M51 (77 square arc minutes) at a maximum
resolution of 5 milli-arcseconds on a single 8 hr pointing with the European
VLBI Network at 18 cm. In total, 7 billion pixels were imaged using 192 phase
centres that resulted in the detection of six sources: the Seyfert nucleus, the
supernova SN 2011dh, and four background AGNs. Using the wealth of archival
data available in the radio (MERLIN and the VLA), optical (Hubble Space
Telescope) and X-rays (Chandra) the properties of the individual sources were
investigated in detail. The combined multi-wavelength observations reveal a
very complex and puzzling core region that includes a low-luminosity parsec
scale core-jet structure typical of AGNs, with evidence for a lateral shift
corresponding to 0.27c. Furthermore, there is evidence for a fossil radio
hotspot located 1.44 kpc from the Seyfert nucleus that may have resulted from a
previous ejection cycle. Our study provides measures of the supernova and
star-formation rates that are comparable to independent studies at other
wavelengths, and places further limits on the radio and X-ray luminosity
evolution of the supernovae SN 1994I, SN 2005cs and SN 2011dh. The radio images
of background AGN reveal complex morphologies that are indicative of powerful
radio galaxies, and confirmed via the X-ray and optical properties.
