Just to update you.
I have disabled the routing functions on the asus routers and put them into access point mode and factory reset them again. I have switched, since 8.30 am yesterday, to Router Mode on the Hub 3. I moved the devices that stream over to the Hub 3 wifi and/or wired directly to it. The BQM after 24 hours:
So it looks much less noisy but the issue with streaming is still there. But now getting problems with streaming between LAN devices too. To my mind this indicates a Hub issue. Going to phone Virgin on Monday or Tuesday about it.
Comparing the two BQM's the difference is stark.
Thanks for the help here.