I had the same 'problem' for a long time and to my extreme embarrassment I discovered that I had the TV connected to the wrong scart socket on the Digibox. Only one of the two sockets has pin 8 control (which changes picture formats), so a change from one to the other fixed it! I think the sockets are marked TV & Videotape or something similar and I had my lead from the TV plugged into the video one.
One final thing, the scart lead must be 'fully wired' to ensure that pin 8 is connected.