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Exactly Alan,
I was going to add that fact to my post above about the roots of lawn-grass being shallow, but wasn't sure if I was right or not.

With respect to the cable in question, the PVC serving on the cable may have split some time ago, you don't know these things when a cable is buried in the ground, it also depends on how low the frost level there actually is.

Alan,
When a underground cable "faults", it can set up what is known as a voltage gradient on the ground above where the fault has occured.
Think of it as throwing a stone into a pond, the ripples that result are the currents "radiating" from the faulted cable.
If that fault does not "clear" (as in operating any protection like an RCD or a fuse/CB), it will cause a difference in potential between two given points on the ground.
Us humans are quite lucky, we only have 2 legs, but when you look at a quadruped like a horse, cat or dog, the chances of there being a greater difference in potential between the front and hind legs is increased.
Also, bear in mind that these animals never wear footwear like we do, a dog may feel a shock where we might not.
Finally, I think what has killed these animals is this, horses wear steel shoes (I've made them myself, my Grand-father was a Blacksmith).
Oddly enough I can't believe they're using aluminium shoes on horses, my Grand-father said (when he was still alive) that they could not adequately support the hooves of a horse and would lead to foot and leg problems.
The worst electric shock a human can get is a hand to hand shock, as it goes via the heart and as we all know the heart is run by electrical signals.
Quadrupeds that stand on the ground would have a situation of "Step Voltage" between each front foot, if there was enough of a potential difference (and current flow) between each front foot, you could kill an elephant, just because of the current running through the heart.

Freesat. A BBC West of England tv newscast today, midday GMT, veterinaries state that electrocution was the cause, not foul play, from a disused cable at least 20 years old. Southern Electric Distribution say area is now safe and cable has been disconnected and removed.

Video clip shows a few guys in SED yellow jackets near a shallow trench no more than 10" deep on the very edge of the paddock. One of them is winding loops of what looks like standard flexible 2.5mm? 1.6mm? pvc non-armored cable easily over his elbow and palm.

Update. National UK BBC News at Six this evening: Another short clip shows the cable to be no more than 8mm [ 5/16"] in diameter and obviously a flex.

So nothing done to any proper standard... even though I know of 40-year-old flex buried underground that's still in good-enough shape not to trip a 30mA RCD, even after winters with very heavy frosts.

Whenever I run underground cable, I cover the whole run with old bricks for good measure, just like I've seen workers do (mostly phone and cable TV).