Ahh, thanks Lahksman, the penny finally drops. Sorry Erel, I see now your link was just an example of a search url.
That does indeed work to a point. Its weak link is that it can only find entries that have been indexed by google, which may have been weeks ago. Whereas a built-in 'search in thread' ability of the forum would find everything including posts only minutes old.
Also I see I need to click the 'repeat the search with the omitted results included' at the bottom of the search results to get all the results