
Well, the readme starts by saying:
There is nothing really much to say here. All you need to do, is install this plugin, then edit the settings (via Plugins > Edit (next to the 'uninstall' option), and update it to your own Search Affiliate ID, so you get credit ;)
A new template has been added, called search_link.html, which is the HTML that is used to show results from SearchFeed.com.
That's about it.
It then goes on to talk specifically about integrating SearchFeed results with search, basically by adding
<%Plugins::SearchFeed_Results::check_results($query)%>
to the top of search_results.html
and
<%if searchfeed_results%>
<%Plugins::SearchFeed_Results::results($query,'10')%>
<%endif%>
where you want the results to appear.
That works - and I can see why, because $query is a parameter in the URL.
[goes away, tries stuff to document what's happening]
ok, I worked out what I need to do - but it's not really in the readme 
in category.html, you need the line:
<%Plugins::SearchFeed_Results::results($category_name,'5')%>
where you want the results to be, and it seems you can do a precheck for SearchFeed results with
<%Plugins::SearchFeed_Results::check_results($category_name)%>
at the top of category.html if you want.
Maybe this is all obvious to everyone else, but it wasn't to me - especially using the $xxxxx instead of <%xxxxx%> version of a tag in this way. My first interpretation of the readme was that I should simply put <%search_link%> in templates to get Searchfeed results.
All is good now though, hopefully these notes will help someone in future 