If you are having Mountain Lion download problems / issues then please read on.
While at a customers site I came across a strange issue. The job was to upgrade their iMac to Apples latest OS Mountain Lion (OS X 10.8). The customer was on Snow Leopard (OS X 10.6).
After starting the App store and the customer loggin in, the download started and then stopped at 79.3Mb. Having tried several times and hitting the exact same stopping point, I restarted the router and the iMac. No change. I then tried the download on my own MacBook Pro, already on Mountain Lion, hitting the same stopping point. I decided to go home to try the download on my own broadband. No issues so nothing wrong with the App Store
In the mean time my customer got on the phone to BT as I said it was worth a try. BT admitted that other customers had had the same issue and the sent a new Home Hub3 router to replace the 2 Wire router.
So if you are reading this post and are having Mountain Lion download problems from the Apple App Store and have one of the BT 2 Wire routers (Pictured below), contact BT and get them to change it.