Replacing a failed Sky box, the "new" box powered up, but gave only 3 channels in the EPG, no possibility to even try to select others.
To cut a long story short, the box (as may other boxes which have been off the system for a while) had an rather out of date EPG, and forcing a software download was the cure, but first, some reservations.
Forcing a software download (by switching the box on while holding "backup") should not be attempted if the signal is unreliable, or if there is any reason to suspect a power failure may be likely (eg. storm conditions), as a failed software download is generally a fault not worth repairing.
And finally, though a friend thought the old card would not work, and the channel set would be the card-less set (no Ch4/5 etc.), it did operate in card/freesat mode once the download reboot had completed.
So, be aware that a Sky box that has been out of service for some time, may require an update. It has also been noted by other guide writers that "dead on next powerup" is also a significant issue with older boxes.
--- Background ---
The reason for this excercise, an elderly neighbour's ex-subscription Amstrad DRX100 had a "standby only" failure after a power cut, apparently not coming back on is a common way for older boxes to fail due to PSU weakness and "bad capacitors" (a company called "Satcure" supplies "reliability kits" which can be used to replace the unreliable capacitors before they fail, but failure may do further damage). This solution was considered, and rejected, as a cure was uncertain. The same capacitor issuses are also a problem on certain computer motherboards, where I first heard of the "bad caps" issue.
A Grundig GDS200/1 box was found at a local charity shop (electrical department, all equipment safety tested), and installed, transferring the old blue/yellow house Sky card.
The Grundig box is also one noted for a coulple of issues, similar PSU faults and also a potential tuner problem which is repairable unless the box is "badflashed" by attempting a software update with unacceptable tuner performance.