... younger eyes I guess.
Nice try PB84. I spotted the paraglider very early on and my eyes are older than yours!
Earlier this year, I was helping to teach ground school at my gliding club and one of my topics was collision avoidance, which made me think about optimum tactics to avoid a head-on.
Our two-seat DG1000, when flying wings level, presents a rectangular target of 18 m x 2.5 m. After rolling into a 45 degree bank to avoid an oncoming aircraft, the target is now much larger - a 13 m x 13 m square.
Since rolling the glider takes time and the collision potential is then higher, I decided that the optimum action would be a wings-level negative-g push, based on the assumption that most pilots will pull and turn.
Of course, my assumption might be false, if the other pilot is one of the students I taught or a PPRuNe reader!
The closest I have ever come to a mid-air was when doing my PPL. Flying west of Reading in a Colt at about 2000', a Hastings suddenly appeared directly in front of us and went over the top about 100' above. No time to react - I imagine the closing speed was at least 250 kts. The big sky theory saved us that time.
A second occasion that is seared in my memory is when I was doing left-hand circuits in a Chipmunk at Cosford, while Wrekin GC gliders were doing right-hand circuits onto the grass to the right of the runway. I had turned onto final from left base, when I suddenly saw a Bocian to my right, not very far away and slightly lower. If I had overshot the turn from base leg, there could have been a collision. I only had about 50 hours then and clearly, my lookout was not very good and there was no warning from ATC.