Thanks for the suggestions. I would like to leave this in place for a couple of days to see if anyone could use the description to work out where the painting could have been taken from (and returned to). Eastern Radar suggests East Anglia, and the Great Aunt in question lives between Sculthorpe and West Raynham. I have no idea how to follow up on the serial number for further info as it does not seem to be a standard format.
M.