Hi Dan,
It happens to sites all the time. In your case it could be the fact that other related sites now have a greater weight/ranking than yours.
Also it _could_ be that you have a good deal of pages (90% of whats in Googles index) that google sees as repeated pages:
site:www.thisishartlepool.co.uk - Google Search
They may well be district pages with their own relevant unique content but google is not seeing it as such
Time to get the fine tooth comb out and go through your pages to check the essentials are there for each page, then think of building your ranking....
Hope that gives you some were to start.