'In Sight of the Surface'

AOPA UK e News for March 2017 includes advice that under Standard European Rules of the Air (SERA) VFR flight in Class G airspace below 3000 ft amsl (or 1000 ft above terrain, whichever is the higher) can take place only where the aircraft is ‘in sight of the surface’. This rule is clarified by the CAA thus: 

“If an aeroplane is flying in Class G airspace at 140 KIAS or less and below 3000 ft amsl, no matter how great the visibility or distance from cloud, it cannot be flown under VFR unless in sight of the surface.” 

The ANO definition of ‘in sight of the surface’ is: 
‘With the surface in sight’ means with the flight crew being able to see sufficient surface features or surface illumination to enable the flight crew to maintain the aircraft in a desired attitude without reference to any flight instrument.”