Dating for yabb
The Register says (and the comments left below now confirm) that Sky tech support don't always tell you how the System Fault can be fixed over the phone, and charge you £65 for an engineer callout on what is allegedly a known manufacturing problem.
The Register has alleged that "Sky charges for repairs to non-faulty digiboxes" (January 2005), and suggested that automatic updates sent from Sky to your Sky box can inadvertantly cause your Sky Plus box to develop a fault, requiring a £65 engineer callout from Sky.
Continue holding the buttons in for about 20 seconds, and when the ring of light on the front comes on, press the 'select' button on the front of the digibox (between the left and right buttons you were holding).
The Sky recording logo lights will spin backwards and, according to the Sky engineer, this will "clear the hard drive".
It would really tidy up the thread and provide closure for an increasing (I guess) number of folks.
It's worth knowing that if you do have an engineer visit (even a free one) your Sky Plus box is under warranty again for 3 months, so any further callouts in that time are free with no quibbles.