Right... here's a clue as to what happened:
I've just been transferring the whole thing from the development site over to my carefully managed live site. The same thing happened - no Announcements... but I expected that.
Then I set about entering them manually again. Finally I went into the module settings and fixed up the template.
Then I clicked on that box to show future updates thikning that the client might quite like that.
This time when I went back to the main page, there were endless copies of the same announcements. Presumably both the ones I exported from the module and I guess also ones that were exported along with the entire portal?
So, that's where the problem lies.. for some reason all of the exported announcements have been dated "Monday, January 01, 0001" when they were imported... making them invisible until I click on that box.
This must be a bug :)
Rob