Re barrel rolling the Varsity - I had it demonstrated to me by a QFI (who should have known better(!) but had also claimed to have done the same in a Lancaster for a much more practical reason) with strict instructions NEVER to do it or even mention it whilst he was still in the RAF. Two weeks later another stude did it and also chickened out and we were all taken to see the result -the airframe was 'fine' but the port engine mountings had failed and the engine was hanging down at about 10-15 degrees!