Thanks for your responses.
I did do some checks to rule out a device on the network possibly overloading the upload bandwidth.
Disconnected all devices, and tried with a couple different devices on their own, with nothing else connected to the router on either wired/wifi. The issue persisted.
This morning I have borrowed a netgear wireless router from a family member. Set the hub to modem mode and checked directly hardwired to the pc, getting full upload speeds, 25mb+ consistent.
Introduced the netgear router in between, and as soon as I did this, upload speed dropped back to 0.5mb. This is with the hub in modem mode, netgear router fed from hub, and pc connected to the router with ethernet. Same story with wifi devices when connected to the netgear router. Absolutely bizarre. After a lot of resetting, connecting, disconnecting restarting etc in every possible combination I could think of I gave up and had to travel into the office to get some work done.
On return this evening, I did a quick speed test, and now my upload speed seems back to normal. No idea what happened in the few hours during the day I wasn't home, but it seems to have settled out.
So for the interim, I appear to be getting around the situation using the hub in modem mode, with my own router providing the connection to my devices.
Don't want to tempt fate, will keep an eye on it, but at least on face value it's working and this is an acceptable solution for me, at least in the interim.
It's an older router but it's a decent one, Netgear X6 R8000 Nighthawk - It's putting out a considerably stronger wifi signal than the Hub 3 in router mode.
Will update if things tail off again.