I'm another who's had the 53.25MB failure issue, and it's 99.9% certain to be a router issue. If you're in the UK then the chances are you're with BT for your broadband. As mentioned above, there is a known issue with the 2Wire BT Business Hub, see link below, and probably other routers too.
http://btbroadbandandhowtosurviveit.blogspot.co.uk/2012/08/bts-2-wire-woes.html
"If you want Mountain Lion, or any other large download, then you'll need to get another router, or firmware upgrade if available." That however can be a pain, and if BT don't play ball then an expensive one. There is another solution however, and one that means no contact with BT, call centres or your local PC World
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What I did was put in an old BTHomeHub that I had lying around to get it passed the 53.25MB point, and then paused the download while I reinstalled the 2Wire BT Business Hub. I then resumed the download and the whole 4+GB was on my iMac in a few minutes (well just over the hour). If you don't have a spare router, borrow one, you'll only need it to get the download passed the 53.25MB stage. For completeness, I waited until it was at about 150MB before pausing.
Hope that helps some.
A2TC
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