def find_meter_by_mpan_mprn(mpan_mprn)
      unless @meters_by_mpan_mprn.key?(mpan_mprn)
        meters = Meter.where(mpan_mprn: mpan_mprn)
        raise DataFeedException.new("Multiple meters found with mpan_mprn #{mpan_mprn}") if meters.size > 1
        @meters_by_mpan_mprn[mpan_mprn] = meters.first