Thanks for your replies ...
Midsstudent - thanks for letting us know (it was working this morning honest!!) - our lookup reference for postcodes not included in our database seems to have crashed - I've escalated the problem to the site owner.
Shewolf - I followed advice from on here and developed some appropriate subject matter blogs which are spread over a number of social media sites all pointing to our site, plus an improved set of keywords for the search engines. Postcode searches are from all over the UK.
xw5 You are right about the hits stats of course - I won't mention them again. I monitor referrer URLs (including BOT activity), entry pages and keyword searches for next layer granularity, and of course the normal search engine clicks, impressions and CTR percentages.