Problems recording from Sky+ onto minidisc

I have a Sony NZ N520 Minidisc and want to record radio programmes from Sky+. All set up ok and recording begins but after 4mins 40seconds the minidisc switches itself off. When in playback mode the player continues to play until I switch it off.

The disk I'm using is new and has no other audio or data files on it. And the player is not connected to a pc, so there are no tracks on it transferred from a computer. This is one reason that the manual states can causesrecordings to stop automatically. No other useful information in the troubleshooting part of the manual.

So I'm stumped! Any ideas ?