def new_log_filepath(self):
        '''returns a filepath to a log which does not currently exist and is suitable for DF logging'''
        lastlog_filename = os.path.join(self.dataflash_dir,'LASTLOG.TXT')
        if os.path.exists(lastlog_filename) and os.stat(lastlog_filename).st_size != 0:
            fh = open(lastlog_filename,'rb')
            log_cnt = int(fh.read()) + 1
            fh.close()
        else:
            log_cnt = 1

        self.lastlog_file = open(lastlog_filename,'w+b')
        self.lastlog_file.write(log_cnt.__str__())
        self.lastlog_file.close()

        return os.path.join(self.dataflash_dir, '%u.BIN' % (log_cnt,));
returns a filepath to a log which does not currently exist and is suitable for DF logging