The online update, MIGHT go to the update folder. In fact that is where you copy the tar update file to do update manually.
Check the folders in storage, You can do a clean up, by using the addon kcleaner. Also, perhaps you need to remove some unused addons.
Me again - ive tried a few things and it is saying that the (fat) partition is still too small to install the update. I don't want to install again from scratch - how would I locate the FAT partition via my Mac please ? Thanks again