Ok, I'm with BT Business Broadband (on an Infinity product, so should be very quick) and my download stuck at 53.25MB... tried various things, including changing the MTU on the ethernet connection, the usual del-restart download, etc. Nothing. Nada. Zilch.
Then I disconnected the ethernet (ie BT Broadband) and used the wifi connection on my Mac Pro to connect via my iPad's 3G (set up a personal hotspot) and the download carried on past the 53.25MB mark.
After it hit 60MB I turned off my wifi and let the download carry on, via the ethernet (so back to BT). It worked. A few minutes later (ie now) it's at 181MB and counting... so no guarantees that it isn't corrupt or anything but so far App Store hasn't complained.
Just thought I'd share that here for all the other BT Broadband users - just use a few MB of our iphone/ipad or even mifi allowance is all you seem to need to get over the 'hump' at 53.25MB. What a PITA! :-(
I'll report back if this works / doesn't - ie if it turns out corrupt or something. Currently I have about 1hr 11mins remaining on the download (while typing it's up to 315MB so I have some hope this will actually work!).
Good luck to all.
PS - note that you either need to disconnect the ethernet cable(s) on your Mac or change your binding order (in network settings) if you want to leave things connected.