Time to sell ?

Wow you have suffered two infamously bad routers: SkyQ (which trashes network packets) and the BT Smart Hub (which is anything but smart).

I recommend switching to a real router, or stick to simple Bluetooth speakers.

I’ve no doubt there are better routers. Can you recommend any to me please. Such a shame Sonos has to be handled like a baby when everything else in my house works flawlessly.

Sonos simply requires routers that can pass data correctly between devices on your local network (few other devices require this, they just pass data to and from the internet). Both of your routers infamously fail to do this correctly. I personally use AmpliFi devices, and they are rock solid.

If you physically increase the distance between BOOST and SMART HUB, does the situation improve?

For my learning: other than Sonos, what kind of systems - home audio and others - need such capability from the router? 

For instance, Amazon and Google speakers do not I believe; even for grouped play they do not talk to each other.

And what feature set of Sonos needs this crosstalk? Apart from the one of not needing internet service to work, once set up?

@Kumar not really my area but some thoughts

Printing, access to a NAS drive are the two most obvious activities that talk internally on your network. I believe SkyQ uses your network to send the TV signal to extra rooms, maybesome Home automation / control systems app control of some of these systems. Possibly some home security or doorbell systems as well, bound to be others. Most of these are not time critical in the way that some of the Sonos Comms are.

Sonos devices communicate to set up and maintain Sonosnet. The most time critical aspect is keeping the speakers in sync in groups and stereo pairs which I think uses WiFi when WiFi connected. And of course the Sonos control app.

I have not tested grouping or stereo pairing extensively with Echo devices, so I can't say if their cloud based way of achieving what Sonos does on these two counts is as robust. But this does suggest that where neither features are needed, router or wifi access point architecture is far less critical than it is for Sonos, for essentially the same return. 
Other than for local NAS play of course; neither Amazon or Google does this as Sonos can, untethered to the cloud. 

If one or more SONOS units are wired to the network, SONOS will build its own wireless network and handle all traffic between the SONOS units. SonosNet does use the same frequency band as WiFi, but WiFi will ignore SonosNet and SonosNet will ignore WiFi. One should make sure that SonosNet and WiFi use different channels and WiFi should use only channel 1, 6, or 11 in order to minimize interference. Your controllers must use WiFi and be able to communicate with SonosNet through the router. I have no experience with your router. Some routers will automatically pick a channel and this channel might change from time to time -- sometimes interfering with SonosNet, sometimes not. Most routers will allow you to use fixed channels of your choice, rather than auto.

I suggest that you submit a diagnostic and post the confirmation number here. SONOS staff can comment on communication issues.

Diagnostic number 664461233

I’ll give that a try ,  thanks