There is a world of difference between a criminal (Police) investigation, and an insurance investigation.
The burden of proof for the Police is "Beyond reasonable doubt", whereas the insurance is civil liabilities and "On the balance of probabilities"
The fact is that you were several seconds and paces away from your door at the time the collision occurred. The collision will have a point of impact, and the Police investigation will work backwards in time from there, so..
Did any of the drivers react to your door opening, and did that reaction result in that collision. Ignore anything else, including witness testimony. That comes later.
If you were three seconds away from impact, where was the car or cars three seconds before?
Every situation is subjective, so would a reasonable person given the facts, say that your door caused an adverse reaction?
Best evidence is camera, second best is independent testimony and finally driver testimony.
You're right to exchange details at the time, you have complied with the law if it's suggested you were a party involved. You are also right to contact your insurers, whether or not you intend to claim.
If you are interviewed under Police caution, my advice would be to go 'no comment' to any questions relating to your actions, and contact a solicitor.
From a civil perspective, there are no black & white's. Your insurer would argue you were possibly responsible to a degree and argue the percentage of that, but that isn't anything to do with you. All they require is your testimony.
If you're guilty, they will require you to sell your Morgan to me at a fraction of it's true value..
