2008 Vauxhall Vectra

I had a new vactra 1.9tdi design. For the first 4 weeks it was fine. Then out of the blue when I was slowing down from cruse control the car started to increase speed. Twice I was able to gain cotrol.I did notify the dealer they gave me a phone nomber and told me to contact gm.
the third time coming off the m4 at bridgend the same thing happened, but this time much worse. The car was totally out of control and driving itself.I hit the roundy at about 70 mph the car was a wright off and have since been in cosultation with vauxall. They refuse to addmitt to any fault whatsoever and say I have to prove it.I have been told that this has happened before. Please can you can confirm if this is so. Also is it true that the ecu will not record a fauls unless the car has been running for a while. I got out of it without a scrat but could have easily been killed. Im a very lucky man. Someone else may not be. Vauxall dont seem to care about the risk to there customers. If you can give me the answer I would be very greatfull and will not use your information yo v/x. This could easily kill people and I am amased at vaxaulls attitude.