@HergestPhil
It would be much appreciated if you could post back with the outcome after the engineer visit.
I've also had my Hub 3 replaced with a Hub 5 and have noticed latency and packet loss issues since. I'm on the M250 package.
From what I have researched so far - it could be due to combining DOCSIS 3.1 and 3.0.
What's concerning is the number of Pre RS errors on the 3.1 downstream channel. I'm nearly at 2 billion corrected errors in a couple of days. You have about 10 billion. I'm not sure if that is due to network issues or line issues?