I wish I'd spotted this event - my school XC runs used to start and finish at the little pavilion just SE of #3. The area around #20 and #21 would've been a mystery to me though, I don't think we ever ran through that bit (and I once went skinny dipping in the river near #17 ;-)
Looks like everyone struggled to find #6
I suspect that the control in the wrong place was actually #5
This resulted in everyone ending up in the unmarked clearing
(marked with the vegetation screen) to the North of the control #6
.