A building site that drags mud onto a public road is expected to have a road sweeper constantly out cleaning a road to prevent the mud / gravel becoming a skid hazard. I’m not sure a farmer would have the same responsibility (as the construction site would probably be required to do this as part of their temporary traffic management arrangements), however it is their mud and so some degree of responsibility must be theirs.
a homemade sign does will not be a highway standard sign. any actual highway warning sign needs to be the correct design, size, reflectivity and mounting height. If the farmer put a sign out saying there is mud on the road he knew there was a hazard but put out a sign that couldn’t have given the same level of warning as a standard highway sign. This in itself would be a worthy point to put to the insurers to justify why you wouldn’t have adequate warning.
in terms of ‘too fast for a country road’ blah blah that people are saying, I have no idea how fast is too fast for this road….and I’m sure neither do they!!