can anyone recommend a decent search extension to implement into MW?

The default installation's query processing is absurdly limited, the retrieval is crippled by bad settings, the relevance is unclear, and the results page is not just ugly but contradictory and confusing.

http://www.askdavetaylor.com/how_can_i_add_a_google_search_box_to_my_web_site.html
edit: this only works for internet based I guess, no good if your wiki is intranet/prviate

I would recommend implementing either XWiki or Confluence to improve your search abilities.