The way forward is to keep getting your broadband ISP to keep raising faults.
"An SFI engineer also said he had seen large HEC and CRC errors at the street cabinet about 3km from the exchange,"
So why didn't they do something about that ?, swap out the E, or load it at the exchange and see if it errors there ?
Yeah that's a good point. If you've called Openreach out on an SFI job, which I assume you would as it's not a voice fault, then the engineer would have run a pair quality test to eliminate any line plant problems? Any faults detected with this would then be rectified by the engineer (9 times out of 10 on the day, so you would hopefully notice an imediate improvement on a fault like this).
All that remains are "exchange faults" as you rightly say, which can include faulty DSLAM ports, which would require a "lift and shift". These are not so rare that it wouldn't be worth mentioning to the next engineer, if you opt for a revisit; afterall he might even appreciate knowing what was done or not done the first time.
Hope that helps