It seems pretty clear that Ian was well of course to the West, for whatever reason.
One possible factor was that on the afternoon in question there was a major wind shift to the East, combined with a significant increase in wind speed. Archive data at the Channel Light Vessel, not a million miles from the area shows this at:
Windfinder - Real time wind & weather report Channel Lightship
Certainly if he had lost nav equipment, which I know from many flights on this particular aircraft functioned well, the changing wind might have affected his flight path.
Last night a state of the art French hydrographic ship spent 6 hours searching a grid pattern in the area. She seems not to have stopped, and then moved on so presumably the search was not succesful.