Hi @cannyboy:
There’s a plugin called Search Unleashed that gives lots of different functionality. One of my clients was using it and I was impressed with the control it gave. Not 100% sure it will give you what you need but it’s worth checking out.
If your blog only has post_types of “post” and you don’t use them for the rest of the site you could use @TerryMatula‘s suggestion but with $post_type => 'post' instead, i.e.:
<input type="hidden" name="post_type" value="post" />