We are potentially in business...I explained everything that had gone before and why I thought it might be external, notably that the guy who came on Friday (who didn't impress me) changed all the hardware and checked cables and said it was fine and fixed...
He said he still wanted to check for himself and is his view, the cable was delivering power outside of the acceptable range. It was only just outside but he said if that's the reading the last guy got, he should have changed it, but it's a fiddly task and so maybe he couldn't be bothered. But it definitely should have been fixed, especially with the issues I was having. So, he respliced the optical cable in the wallplate a couple of times until he was happy with it - reading went from about -26.7 to about -19, with the acceptable range being above (below) -25. So, he is convinced this will fix it. It's only been a few minutes since he left, but the speedtest readings are already better than they were before (even when it was "working") so fingers crossed we are in business.
More than that he actually listened, didn't dismiss my testing and comments and explained why and what he was doing, which goes a long way to great customer service! He said he will personally message me on Friday to see how it has gone until then. Fingers crossed!!