@Lee_R - thanks, I really appreciated this - the engineer was on my doorstep the very next day!
So here's what we found:
When I downgraded my TV package and lost a box from upstairs, that should have been a visit from an engineer to change my 1-in-3-out splitter to a 1-in-2-out one, but that never happened (was just told to unplug and return the box and that was that). As things were, I had the signal split to a box upstairs that was no longer there - it was likened to a leaky tap, where pressure (or signal) is being lost in the system because it's not capped off. So that was changed.
Then there was some fiddling in the cabinet outside due to my upstream power level not being quite right. I suspect that this is what @risc19 was concerned about.
Finally - it was established my Hub 4 wasn't responding as expected, so that was changed for a new one.
Would like to recognise James (the engineer) for his efforts, if that can be passed on in some way.
As of now, I'm achieving ~630Mbps to my hub. Honestly, it's better but I'm still convinced that it isn't fast enough, all things considered - remember, the first week after install I hit the best ever recorded speed of 875Mbps, so I know that the existing infrastructure can deliver better than at present.