The engineer came on Friday. Made some adjustments in the street corner box and added an attenuator on our hub, which has corrected the upstream stats. We've had no issues since then.
He couldn't advise on why the hub kept reporting it thermal throttle errors. Confirmed there aren't any fans in it (which is why I'm never hearing and fan noise!), but also that the hub 5s are very new and he hadn't seen many of them - and didn't have a spare. Agreed we'd keep an eye on it and raise a new ticket if we get further issues.
All in all I'm happy with this - the box is working and seems to be back to reliability.
Dan.