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
At FL100 over a 3000 ft overcast, how much surface do you need to have visible to maintain attitude without reference to instruments?
Conversely are UK PPLs prohibited from night flying on long over water crossings or remote terrain (i.e. where you have no visible horizon or surface below you)?
[SARCASM OFF]