The image now also has appeared on Diptera.info, submitted by Chris Raper and he already attached the name Macquartia to it. The image of Macquartia dispar in the Diptera.info Gallery seems a good match (http://www.diptera.info/photogallery.php?photo_id=6020). If it turns out to be another we will report back.
That's great - really appreciate the help. As a beginner I was amazed to see the diversity within the Tachinidae and suspect that I've been unconsciously lumping them together with the muscids and other superficially similar families. I'll try to get my eye in and learn the differences at family level as a starting point!