I have created an online interactive probability creator which you can find here. This is still formative and feedback is welcome. Eventually I want to make it so that you can save your own settings. It is difficult to find items of evidence that incriminate Robin. While doing it I realised how incriminating the blood on the back of David's T-shirt is. How on earth did he get that by going round inspecting the bodies!?
The other item that struck me was the trigger lock key. I understand that Robin already had the principal key in his possession, well, at least in a coat in his caravan or somewhere, as a result of keeping while David went for a swim the previous day. Was it determined that the spare key was used to unlock the trigger? Was the principal key still on the necklace in the pocket? If Robin had wanted to commit the murders he would not need the spare key since he already had the principal key. Any enlightenment on this by experts out there would be appreciated.