Fixed this today.
Sky+ was stuck on standby, and the firmware upgrade (hold down backup while switching on made no difference).
I stuck the hard disk into a PC, downloaded the excellent Copy+ from http://www.skycopyplus.co.uk/, and backed the disk up (to a 260GB file). Of around 500 programmes, Copy+ reported problems on around 20 of them, so I clicked on "No - don't copy these".
You then need to remove the partition on the Sky disk, because Copy+ will only write to a clean disk. I used Windows' Disk Manager to do that.
Then I restored from my backup file. Put the disk back in the Sky unit and had an anxious ~4 minutes while it did "something" and then, tada!, it sprang into life.
No idea what caused the failure in the first place, but this method saved me losing all my recordings.
It's not a task for the faint-hearted, but if you're handy with a security Torx driver, and IDE jumpers, you'll be fine.
Scott, December 2009