I previously had the 350Mbps package on the hub 3 and would consistently get download speeds of around 380Mbps.
To rule out other causes I've checked the cabling, disabled both wifi services, switched the router to modem mode, reset the router to factory settings six times re-testing each time, rebooted about 25 times etc, but the problem persists.
Would you mind reviewing my router values (pasted below) and commenting if there's anything unusual looking?
I don't think that's acceptable or a reasonable explanation at all.
But an accurate one nevertheless. You don't have a dedicated line, there's nothing outstanding in the hub's diagnostics pointing to a problem and the speedtest result from the SamKnows site all indicate that the c. 800 Mb/s download you are getting is the most that the local VM infrastructure can provide - although I have a healthy dose of scepticism regarding the accuracy of the speed test results and the hub's logs.
I believe that the minimum acceptable speed on the 1 Gig tier is somewhere around 550-600 Mb/s, so VM will probably claim that what you are getting is above that and is 'good enough' and doesn't count as a fault.
If you are still within the 14 day 'cooling off' period, then you might want to think about reverting back to the 350 tier for less money or even the 500 Mb/s which will cost you less than what you are paying now.