John Rumm wrote:

Got a call from my Mum, who is currently having a fair amount of
building work done on her place (large Victorian semi,

Remember that last word:

.................................... she mentioned that a plasterer
was complaining of getting electric shocks from a wall!



My first thought was that if he was getting a shock like that, it was
surprising the RCD did not trip. So I did an RCD test, but it looked
like it was working fine.


Any thoughts?

The RCD seems to work OK. The "significant belt" you say the plasterer
got with a wet wall may've been below the tripping threshold. But a
thought which runs through my fevered brain is that the source of the
belt might be from next door - t'other side of the party wall - which
need not have an RCD in place, but might have sthg in contact with (say)
the old lead pipes you mention.

Just another confusing possibility to add to your mystery - Stefek