Sadly you are best putting this down as a lesson.

People tend to drive on roads they know well more quickly as they are familiar with what is coming next - or think they are, and so when it does go wrong they are going to fast to sort it in a tight country lane. So they tend to drive to fast for the conditions of the road at that particular time.

20mph? lets be honest, that would be a snails pace and it is unlikely that was the actual speed, and if your misses is agood a driver she would have easily brought it to a stop.

Yes the farmers shite all over the road and a dated country road design is not the best combination, but all the more reason for people to drive with caution, drop the speed to the conditions and expected the unexpected.

My mate lives on a similar type of road and the locals motor it down there all over the road cutting bends etc, and surprise surprise there are countless bumps and accidents on the bends.

Nobody likes to put their hands up and say yes, my fault, could have been avoided, but there is little point trying to prove a farmer shite or the councils road design is the cause of the bump. Better to just think 'bugger' and move on.