It was no more than the Boks' second string deserved, after engaging in a full-blooded tussle.
Demas, who scored for the Cheetahs against the Lions 17 days ago, finished with a flourish from arguably the game's most flowing move.
The elements did their utmost to blow both teams off course - combining rain, hail and gale-force winds.
But a first-half try by full-back Keith Earls - plus a penalty and conversion from captain Ronan O'Gara - ensured that as in New Zealand four years ago, the Lions' midweek XV remained unbeaten.
There were also a few interesting selection pointers for head coach Ian McGeechan for Saturday's second Test appointment with the world champion Springboks in Pretoria.
Wing Luke Fitzgerald shone in most areas, while number eight Andy Powell displayed a healthy appetite for battle and substitute James Hook - on for O'Gara just after half-time - made some assured touches, including slotting a late penalty.
The Lions, though, experienced a testing finale. They were unable to put daylight between themselves and well-organised opponents - and they ultimately paid the price.