There is actually no statutory depth to which cables should be laid although there are various guidelines, so it comes down to common sense if this gets to court.
Any claim for injury needs to be reasonably foreseeable and that could be problematical here. However although this was frightening I am sure, you weren’t injured. BUT it would still be worth consulting a personal injury lawyer who may want to take it on if they can prove that you suffered some kind of psychological or psychiatric damage (PTSD) from this. I think it unlikely but it’s worth a few phone calls.
I think the best you could hope for would probably be to get 50% of whatever you are claiming on the basis that you are partially responsible for not investigating 1st to see whether there were only underground cables. I know that you would not normally expect them in this situation but nonetheless, you didn’t look it seems. That is the view the court would take.
However, the question was not about injury but was about the cost of the cable and I mention the injury because it may be a way of getting the electricity company to drop the matter or at least drop part of it.
My suggestion would be to put it to the electricity company that they have been negligent in placing this cable where they placed it and that accordingly, you should not be responsible for the cost of it. Tell them that you believe that you have a claim for personal injury as a result of the negligent placing of the cable.
If they are still unmoved I would either let them take me to the Small Claims Court for the cost of doing this or offer to give them 50%.
If they do not accept 50% I would let them take me to court for the whole lot and let the judge decide whether they are to blame or you are to blame.
Can I clarify anything for you?