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My first suggestion would be to try another browser and see if the problem is with the site or your browser. If it's your browser, check for an update of both the browser itself and any extensions you have installed.
I'll try that tomorrow and let you know, but yes, cookies are enabled.
Also, I tried c+ping the address of the final page of the download over from IE into FF, and it would quite happily download, it just won't redirect to the pledge page from the blank download page.
It could be something to do with the cookies though, since it was asking me to log in an abnormally high amount of times. Thanks for the advise!