The sparky is responsible for any damage HE causes while on yr premises working.....its that simple, from plaster damage to foot through the ceiling..to nailing a water pipe..to creasing a gas pipe to carpet damage...(ive done all these!!)
Legally, he's responsible for taking reasonable care. That's to say, not making avoidable, or unnecessary damage - like nailing water pipes, damaging carpets, and so forth.
He can reasonably be expected to know that there is a gas or water pipe in his work area, by doing a fairly simple check or two. He can reasonably be expected to know that dumping half a wall of brick, plaster, and other debris on the Wilton or Axminster might render it damaged. He can reasonably be expected to know that sawing his trunking on the dining room table is probably going to end up with chunks out of it.
Just the same that he can be reasonably expected to know that failing to connect the CPC, for example, might render the end user liable to shock.
However, if something happens that he cannot reasonably be expected to know, i.e. he sinks a chisel into a chase, and the wall falls down (he's not a structural engineer), or the plaster blows, for example, then it becomes force majeure - an act beyond his control.
The matter here, is then one for negotiation, as to responsibility - an electrician will undertake to sink a chase into a wall based on certain reasonable assumptions - that the wall is sound, that the plaster is sound, etc., - and arguably, it is for the premises owner to know, if that is NOT the case - it is their wall.
Your argument assumes that the electrician is responsible for the condition of the wall both BEFORE, and AFTER, his work. Not feasible, or reasonable.
The upshot in almost all these cases is arbitration. Mostly, as indicated in so many other posts, the electrician will be sufficiently bothered about the overall quality and impression of his work, his reputation, as to repatriate the wall to an extent, to minimise the perceived damage.
And even if he isn't, sadly, it is unlikely the courts will take the view that he absolutely must make good any consequential damage for which he could not reasonably be held accountable, assessed much the way I've described.
Again, I stress that whilst that is the law, one would hope that reputation, and desire to keep customers happy, would prevent such a case ever going that far. As I said before, for the sake of the cost of a bag of plaster, or tub of filler as required, and a half hour chunk of labour, I'd make good, and hope that it kept that customer on side for the future, and for the future of whomever they may talk to.
