I've been following this sad tale with interest as I was flying a glider in a competition THAT day and passed through THAT area of the collision only a few minutes before the sad event. Indeed, I heard another pilot report seeing the collision on the radio and there but for the grace of whomever, it could very easily have been my glider rather than the Cirrus that was hit.
To put things into context,this is a VERY busy piece of sky and my logger trace is one of those included within the AAIB report identifying the choke point that others have mentioned. The day in question was also a VERY good day as I recall, with a cloudbase of circa 5,500ft, strong lift, cloud streeting and seemingly anything with wings taking to the air that day. With all this in mind I must question the wisdom of conducting aerobatics in the area that they did.
As an instructor, I like to think my look-out is good, but I can honestly say I wouldn't have been expecting aerobatics there and indeed when I flew through only minutes before, I wasn't especially looking for aircraft doing so and rapidly climbing from beneath me.
A sad event all around though and especially so when it seems that it SHOULD have been survivable.