Refuse to accept the blame on any account, take lots of photos of the area if you can and sketch a diagram of what happened.
If it comes to it you will need to provide evidence that you weren't at fault even though it seems obvious. I presume you were alone in the car?
I'd call your insurance right now and ask for advice on what to do, if you don't get a suitable answer then I'd seek some professional advice outwith internet forums.