SKY have introduced a new system update in the software on your DigiBox. It is the addition of an auto-standby feature, in order to appear to be "green" minded. All boxes have been updated via the satellite link, but there is an option to disable it.
Follow these instructions to disable the 'auto-standby' feature...
Press the "services" button on your remote control
Select option 5 (Auto Standby)
Use left/right arrow buttons to select 'OFF'
Use up/down arrow buttons to select "Save New Settings"
Press 'Select' to confirm the new settings
Press the 'SKY' button to return to normal viewing.