Have done advanced search (hidden/system) for memento in config folder and then entire C drive for mention of memento - and drew a blank.
I think the logical thing for me to do would be to start introducing the extensions into CS4 one at a time - this should replace the edit > WebAssist Preferences on the edit menu (hopefully) and I can presumably disable it then.
I may need help getting the extensions loaded then as I had some persistent problems before, but if and when I get problems I will post back in a new thread.
Thanks for getting back to me.