I'm a gliderpilot, and the importance of a constant lookout is drilled into us
That's a nasty comment. The implication that the pilots involved were not keeping a good lookout is reprehensible. The ONLY people who know whether or not this is true are those involved and, sadly, they cannot tell us. What makes you think pilots of powered aircraft don't have the same or even better training? I certainly did and it was commented on when I had a check flight for another club. Please refrain from trying to score one-upmanship points on such a sad thread as this one. We should all have respect for those who have died and their families and friends and leave the discernment of the cause(s) to the AAIB who have far greater skills at determining the reasons than anyone here - except their investigators should any of them post on/read PPRuNe.