One of the great things about Ubuntu is its automatic updater thing. Every now and then it checks to see if there are any updated versions of anything you've got installed. If there is, it gives you the option of installing them. This means that everything is always up-to-date. I often give this as an example of a practical reason for why Ubuntu is better than Windows.
But the other day, the updater suddenly stopped working! Each update is digitally signed so that you know it's really from Ubuntu, and for some reason my system wasn't recognizing that the updates were properly signed.
The way round it is to open up Synaptic in the System > Administration menu and change the repository from UK to Main. It must be that there was something wrong with the local UK repository. I hope this helps anyone else who comes across this problem.