FTTB Broadband Speed Issue - Any Openreach Guys?

This is the situation; I live in a medium sized town in the south of England and have paid premium rates for fibre broadband since it has been available. Back in 2014ish I signed up and had an initial download speeds of ~65Mbps, I never had any issues with connection dropouts or speed, three years ago we moved house literally across the street (from no.3 to no.7) and all was fine, speeds were a little lower ~56Mbps but I understood the variables involved and accepted it. No changes were noticed within the first year, around about this time I changed the ISP from PlusNet to Sky, this is something I did not want to do as I was perfectly happy with PlusNet but the wife wanted Sky TV and it made economic sense. Shortly after this changeover I noticed the connection speeds drop gradually bit by bit, I informed Sky who did some tests and assured me there was not a problem but they carried on dropping to ~20Mbps or so, Sky sent a couple of engineers and then basically washed their hands of me and I rejoined PlusNet.

At first speeds were back up to 55Mbps, but not for long, I have since had a guy from Openreach who said he had found a break in our box and repaired it and assured us service would return to normal, he also told my wife the speeds promised were to the box and did not include the 'last mile' on any network which I know to be a blatant lie. I am an engineer myself and I understand at times you have to 'cover your arse' when things go awry but this is proof enough to me there is a problem that he couldn't repair. There was a minuscule improvement of ~3Mbps for 24 hours then speeds plummeted once again to below what they were before the engineer arrived.

This has been going on for two years now and I am appealing for help in understanding the details about my connection to the box, what problems could occur in a cable that might not be able to be detected, layout of telecoms infrastructure within a early 1990's housing estate, procedures the ISP might adopt like throttling etc. I do have the feeling I am being fobbed off and feel I would be able to appeal to a higher authority with a better understanding of the technology.

Thanks you in advance.