Hi Graham
A quick look at your anchor text tells me you have an over optimization issue
--> http://screencast.com/t/lwzNZgEu5X
The very high percentage of commercial keyword anchors linking to the site is why the site used to rank well, and now this is not helping sites anymore or even hurting them.
I have seen this many times now, where sites with less authority but more "natural" anchor text perform better.
There are some options;
- You can request these sites change the anchor text to the brand name or domain name
- You can disavow the links (NOT recommended unless done with a super amount of care).
- You can do step one AND work on building newer more natural links
When Google sees those commercial anchors they know some form of link building has been done, and it gives a negative mark on the site - now sites like that have to work hard to earn Google's trust again.