Quote: Is it reasonable to assume that these are the Aerostars Yaks that are involved?
No!
There is a formation flying school at North Weald every April and October where Yak owners can get skilled training in the art of formation flying and aerobatics. I almost joined one a few years back when I was Yak owner but free time was not available.
I still know some people who have taken part in this and now instruct.
Thanks for confirming that. Wasn't aware of the FFS - didn't know of any other flying group of Yaks apart from the Aerostars.