I've come across this type of thing before. The DNO replace the old overhead supply with a nice new underground one, but don't bother to install a main earthing conductor from the cut out to the consumer unit.
I would carry out a Ze test on the TN-C-S supply and providing it is satisfactory...
There is no requirement to be part p or an approved contractor to carry out domestic pir's.
You only need to be competent!!
Once you have completed any pir's you then may need to be part p to carry out any repairs.
Sounds to me like you have charged up the armoured cable going down the garden and got the shock off that.
Don't forget that an IR test is DC and this effectively turns the cable into a capacitor, holding the charge. (until you discharge it)
Mine is a square D. 6 way with an RCBO on the ring only (1 ring for the whole house) haven't got time to upgrade, too busy painting and decorating, mowing the lawn, claning the gutters, and doing all the other things that the missus thinks look pretty.
If you need the drums in tact to reel them off a cable drum holder it is possible to repair them.
This is hard to describe but I'll do my best..
you will need 1) the sides off a used cable drum.
2) two bits of wood (2' x 1' and about 10 inches long)...
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