Thank you Brian for that. It sent me back to look at
http://aufhimmelzuhause.com/id101.htm and that account of his mission on 23/24 August.
It would seem that the Lancaster that he shot down landed near Shouldham which is Cassini Grid Square approx G21 and I wondered if the Crash observed by Rad & Lovestad at G81 might have been that and not the ME410. Schmitter's account suggests that they were attacked near East Anglia and struggled to fly back towards their base at Gilze Rijen and not being shot down near Zeebrugge.
If the former were the case there would perhaps be two RAF claims for the same German loss and the fate of Matheson & Bush would once again be unknown.
I suppose at the back of my mind I am trying to determine where their earthly remains might have finished up. The question of a friendly fire option was never disclosed to the two surviving people in 2006/7 for whom it might have been upsetting. Viz Matheson's brother and Bush's widow.
Sometimes there just isn't an answer :-).
Kind regards,
Howard