Hi and welcome - hope you don't get bitten as you did on the other forum.
http://www.rafcommands.com/forum/showthread.php?t=6646
From the information you have so far gained, it appears that Rowland (later Chief of Staff RAAF) piloting PB228, stated he had been hit by another aircraft (which also crashed at Grossauheim) As his aircraft broke apart, it is possible that it did fall in different areas, coming down from 17,000 feet at 200 mph..... The wreckage landed on the bank of the river. The crew don't bale out in a nice neat group, it's every man for the hatch and obviously the pilot tries to retain control for as long as possible.
What now makes you think it was lost to flak? I don't know what more information can be discovered, or about your grandfathers brother, Leslie Morgan, the navigator on NR195.
Like you, I'd love to access reports from flak batteries, in my case those aorund Leipzig on 4th Dec 1943....
We'd all like to help you and outdo the other forum, so what can we do that you haven't already got? The flak records were virtually all lost or destroyed in the chaos at the end of WW2. Those that were captured were taken to the US where they remain and very little has been released about them. Why? No idea, can't see it's protecting any big State secrets after all this time, but you know how Governments love their secrecy!!
It's frustrating because night fighter records are available online, so if an aircraft doesn't appear as a claim it's likely to be flak, weather or accident related (and generally in that order!!