Nick: "It also ranks as highly across all the other search engines too which I would guess is down to its google position partially."
Oddly, what works for one engine doesn't always work for the others, and it can be hard to balance them. Most of my clients' sites rank well in Google, Yahoo and Bing, but there are a couple that have top Google positions and fair Bing results, but which Yahoo seems to dislike as they perform poorly in it.
Depending upon whom and when you ask, Google has 70% - 80% of search market share. If someone can choose just one engine to optimise for in order to hit the biggest audience, Google is the one.