It must not happen again, but it will, nothing is as certian as death and taxes. We just have a responsibility to learn and move on so that those that died have left something behind.
Plussix, you are emotionally involved and will always maintain that Yak UK were at fault as it will help you come to terms with the loss. We all deal with loss in our own way I guess. Personally I think there are so many grey areas in this case that I would not try and blame anyone but learn from it which I suspect a number of people will have done so. Yak pilots will be extra vigilant in pre flight checks and the maintanance organisations might just sharpen up as well. So as I said a senseless loss may help those that come after to prevent the same happening again. A worthwhile legacy?
As far as the screwdriver was concerned I would have been in the camp that if I found it in my aircraft I would have kept it. I found a spanner on the top of my engine once and it now sits in my tool box.