.... we have all seen missed errors, and the question arises if the errors missed with a PIR/EICR can one claim from the report provider? Or at least their professional indemnity insurance.
Claim for what? The remedial work would have to eventually be done (once the need was eventually discovered), and paid for (by someone), regardless of whether or not any PIRs/EICRs during the period had failed to detect the problem.
I suppose one could complain (to the report provider and/or his/her CPS, if there was one) about the incorrect report which one had paid for (maybe hoping for some 'refund' because of the unsatisfactory service that was provided) - but, beyond that, I see nothing for which one could 'claim' from anyone who had provided a PIR/EICR.
There are two options to correct, one is the supply side of cable, the other the garage side of the cable.
Indeed - and, as I've said, I would suspect that it would be easier (hence probably cheaper) at the garage end.
But it really is down to who pays, if one party is satisfied to just not have power to their garage, then no one can really force them.
It may be unavoidable, but neither of the current house/garage owners should really have to pay. I have no idea of the legal situation but if (as seems to be the case) the company which undertook (or, at least, sub-contracted) the original installation still exists, then, morally speaking, it should be for them to correct their error, without charge, shouldn't it?
In a situation like this the supplier (or suppliers) involved may have something to say, and may want to 'get involved' - since, as a result of the error, two customers are being charged for electricity being used by someone else. As I've recently suggested, they might well both end up paying roughly the correct amount, but that's not inevitable, but the supplier(s) may nevertheless still be unhappy, 'on principle'