Can't find a reg on it, but it just seems wrong...

[Mark42]

...
The stables were supplied from a cable I don't recall seeing before (see picture). It is a 20mm almost solid pvc cable with a solid core conductor. They had used one for the live and one for the neutral so there were two cables running the 40m to the stables.
...

As Brianmoooor said, that's electric fence cable. From the look of it, it's the cheap stuff which has a galvanised solid steel core. The CCA is usually about 1.5mm. Although insulated to HV levels, no way is it suitable for LV ac mains. It was probably found in a shed.
I'd say replace the whole installation, or walk (run?) away

 

[Rockingit]

Something about how you can't make a horse drink? I attended a house years ago after a "there's flames coming out of the wall!" kind of emergency call-out and after some digging around I discovered a previous electricians report folded behind some pipes near the ancient distribution system that had basically condemned the place the year before - I asked the owner about it and they said that had been left after his (adult) son had been electrocuted whilst using the shower..... sometimes you just can't cheat Darwin!!

My advice in this would be to take a line with the customer that says it'll be cheaper for them just to replace the whole lot rather than wasting time having you find every last fault first and still ending up in the same situation.

 

[Pete999]

I had an IR <30 Ohms between R1 and Rn and zero Ohms between R1 and R2.
There was definitely evidence of degridation but that's not what I was asking.

What type of meter did you use to get these IR readings Martyn?