Tephritid on Knapweed

The attached images are from a site in Worcestershire, taken in mid August and have been sent out from the Wildlife Trust for ID.

As I'd attended the Dipterists Forum course on Picture wings at Preston Montfort I thought I'd have a go. 

I think it is Chaetorellia jaceae from the draft keys given out by David Clements. Is it? 

Carol Taylor