Bob,
Yes Thorpe Abbotts was near Diss in Norfolk some 45 miles from Alconbury, again on Fen flat land of East Anglia, basically the big bulge on the UKicon map.
Accidents appear to have occurred more frequently on large bomber formation attacks. This also happened in UK Squadrons as well.

In terms of bombs being dropped on friendly aircraft below, imagine the scene, you are over your target, everybody is concentrating on "seeing" the target in all types of cloud cover,weathers and of course night time. Hoping that in the latter that the target had been illuminated by the Pathfinders dropping their flares.

The very last thing you would be worried about, would be another friendly aircraft above with its throttle slightly advanced dropping its load on top of you. Interesting when you consider how many people, other than the bomb aimer who takes control at the last minute, didn't see the disaster unfold as planes accelerated above!!