I've just noticed that new images uploaded via 'Add Gallery/Images > Upload Images' works fine for the large image, but thumbnails are not being generated (the thumb's show on the gallery page as blank boxes with the filename in them). On checking, the process goes entirely as normal as far as the point when the info box says "Generating thumbnails", it goes up to 100% and then seems to hang there and the box has to be closed manually.
For info, images uploaded via the NextGEN public uploader work fine for generating thumbnails, albeit only one at a time. However, I've just installed NextGEN GalleryView plugin, which I *believe* just upgraded my jQuery. I've disabled that plugin but the problem still exists - I only mentioned it in case the jQuery bit might be important in finding out what I've done ;o(