thanks, 9G and Penko
Many thanks for all the inputs. I now realise that "check ILS Course" means exactly that and NOT drift angle between heading and track.
The 5 deg course difference limit is probably required to ensure a successful autoland in case of a loss of LOC signal after flare (the AUTOLAND light will not come on for this eventuality below 15 ft). In this case the aircraft is going to align ground track with the IRS (runway) heading. So if this is off by any number of degrees, you would touch down with that amount of drift?
Your thoughts?