I had an issue with BT a couple of years ago where my speed (Infinity 2) dropped from 80Mbps to around 55Mbps over the course of a few months. I logged this with BT support a couple of times where I was helpfully told to reboot the Homehub I wasn't using.
Long story short, I moved to PlusNet and queried the speed I was getting which they agreed didn't match the profile my line should have been capable of. Cue three visits from Openreach engineers.
The first one said everything checked out and left - no improvement.
The second one said everything checked out, replaced the master socket and left - no improvement.
The third guy had been to a wedding the previous day and I think was nursing a hangover. This one did a full end to end line test, disappeared off to the cabinet, came back, did more testing, went off to the cabinet again, sat in his van and ate sandwiches, went to the cabinet again and then advised the port on the line card in the green box was knackered. He moved me to a different card and I was back to near enough 76Mbps.
It took about three hours and I still the guy is a legend.
All the time he was commenting about how Openreach don't give their engineers the time to diagnose faults properly before they have to be on to the next job so the experiences listed here and in the article really don't surprise me unfortunately.