Well we have seen no problems at all until this monday, when traffic plumeted. Site is over 10 yrs old and has had a stable/rising presence in the ranking for all the time until this monday.
Having spent most of today trying to work out what's going on, I still have little idea apart from the fact that it seems that google are having problems with anchor text.
Looking at a number of searches to see who has replaced us, if I look at the cache, I find that google states that some terms "only appear in links pointing to this page". But the page is only vaguely related to the search so it seems very unlikely that the terms would actually exist in any link to the page.