Hi mattv, did you try with Chrome or Firefox? This happened to me with McAfee Agent when the package was corrupted but in your case thist would mean that the whole repository has been corrupted and that's not very usual.
Laszlo, thanks for pointing me in the right direction. Strangely, all of the packages in my Master Repository had somehow become unchecked (which is unusal enough on it's own), but easily remedied. Everything is working fine now.