For those who are interested this is the sort of cable that's usually used for direct burial in the UK:
There's a lot of paper insulated, lead covered still in use for distribution, along with the more modern XLPE insulated stuff.
For the OPs problem I would probably either hire a cable tracer or just dig at the end to see what direction the cable is going and see what can be deduced from that - cable runs are usually pretty straight. Most commercial cable tracers inject modulated 33 kHz and then look for that, a few rely on the cable re-radiating broadcast LW signals.