CDO

I'm trying to remap a 2 dimensional field with 2d lat and lon coordinates to a regular 0.5x0.5 degree field using

cdo remapbil,r720x360 filein.nc fileout.nc

but am getting the error

cdo remapbil (Abort): Unsupported grid type: generic

I've added the lat and lon to the coordinates (which seems to be the first thing that comes up with a google search of this error) so the file contents (with a ncdump -h) are

netcdf filein {
  dimensions:
    i = 360 ;
    j = 270 ;
  variables:
    double barotropic_streamfunction(j, i) ;
        barotropic_streamfunction:coordinates = "Lon Lat" ;
    double Lon(j, i) ;
    double Lat(j, i) ;

// global attributes:
        :history = "Mon Mar 26 17:18:23 2018: /project/ukmo/rhel6/nco/bin/ncatted -a coordinates,barotropic_streamfunction,c,c,Lon Lat ECCO_Btp_streamf_avg_small.nc" ; 
  }

but it's still giving the same error. I'm attaching the file - any suggestions of how to fix this please? Thanks