Actually, scrub what I said before. The LSO players dotted around the LSO On Track orchestra are not really bumpers (ie reinforcements to add a bit more sound). For one thing, there’s a fair amount of talent here, in every section, that make you think they could manage quite well on their own. (Although, of course, getting to play next to their LSO mentors is part of the point.)
For another - ”more magic”, “vibrato here on every note, not just one in six” - they are taking an extremely active, vocal role in urging their young colleagues towards higher standards. Or there may be some words of technical advice, or a problem fixed between members of a section.
But it’s when the rehearsal takes a break to watch a short film about their Olympic exploits that the rapport between LSO players and the kids really comes across. As mischievous jokes are shared. it’s hard not to feel that at least some of those grown-up players wish they were back in youth orchestra, larking about, again.