I have not flown a Nimbus 4D, but have several hundred hours experience in
its' early predecessor the Nimbus 2.

A couple of points that might be relevant to this accident:

1) With flapped gliders there is usually a positive flap limiting speed,
often way below Vne. If you are thermalling you are likely to be in a
positive flap setting, so if a loss of control occurs that leads to a spiral
dive, it is very easy to exceed the flap limiting speed and risk twisting
the wings off.

2) With large span gliders, opening the airbrakes causes the wingtips to
bend up. We fitted a second paddle to our Nimbus 2 airbrakes and I did the
test flight in which I was supposed to fly up to Vne (135 knots) with the
brakes extended. By 95 knots the wings were bending up so much that I
started to fear for the structure and didn't go any faster! I resolved that
should I ever lose control of the speed, I would slow it down by pulling g
rather than opening the airbrakes. It also had a tail-chute that could be
deployed in extremis.

I can only recommend that the first action in any loss of control situation
should be to select neutral or negative flap, and then sort things out.

Derek Copeland