I have gone through a couple of ideas, some of which I've discarded. For a start I think the issue is recent, or we'd not have survived the winter. So first suspect has to be the cable uprooted by ploughing, although it doesn't look too bad the outer casing is damaged at one point and slit along its length. That's been reported to OR so we'll see whether they do anything.
Second theory, based on such a rapid response, was that it was more likely to be overhead rather than underground. An underground joint being unlikely to get wet the moment it starts raining, or to dry out so fast either. Our main cable runs along the public road to a pole which I think counts as our DP, it's a black underground cable going up the pole. Two thin wires cross the road to another pole on our side, and the grey 5 pair (?) cable comes down that pole to start it's 1/2 mile journey to us. Looking at both poles with binoculars the sheath on the grey cable is split and cracked in all sorts of places, which I guess must be old age rather than impact damage.
Between the pole and ourselves there are maybe 8 or nine joints, the accessible ones are all the resealable clip together sort. None look damaged, although one the lies on the surface was driven over by a tractor recently so would be worth a check, meanwhile I've covered it in a plastic feed sack and tied it to a wee post.
A slight academic oddity is that the errors this morning were no accompanied by any apparent change in noise margin.