You sent email as '@northantrim.org', so you should generate DKIM key for domain 'northantrim.org ' and add DKIM related settings for this domain in Amavisd config file. Also, append 'northantrim.org' in @local_domains_maps please.
If it still doesn't work, please extract Amavisd log entries in Postfix log file, check whether there's error message or not.