I suspect the fixmystreet issue is because it isn’t a street (ie not an adopted road) so probably gets ignored. Most of this stretch is a bridleway with a bit of undefined stuff as the northern section.
Nonetheless it is the landowner’s responsibility to sort out dangerous surfaces on bridleways. Whether the landowner is the County Council or not, they should either act or sort out who should. Current advice is:
“contact your local highway authority (which will be the County Council, Unitary Authority or National Park Authority), informing it of the situation, preferably by letter or email”