Sounds like some sort of wifi interference with your device that you’re running the S1 controller on, since the Sonos seems to be continuing playing. 

Might be helpful if you were to provide more information about the nature of your network, and what you’re running the Sonos controller S1 version on. 

It's BT wireless Wi-Fi which is cloned around the house using 3 TP link power line adapters. The devices that the Sonos app are on are an iPhone 11 and an iPad Pro. The set up has worked fine for a couple of years until recently 

Whilst you may not have changed anything, BT may have sent out an update that’s changed a setting somewhere. Or your neighbour may have a new wifi network. 
Maybe rebooting everything may clear/reset something? 

Your phone is likely jumping frequencies (2.4-5) and your router or mesh system is too dumb to handle broadcasts between them. Network infrastructure issue. Have you followed the official Sonos recommendations for BT routers (which are often janky)? (Those TP link power lines may also be contributing).

My daughter/son-in-law have a BT router and it’s been a real nuisance from day-1. I’ve advised him to ‘bridge’ it and just get his own router instead.

It appears some of their router models are real ‘flaky’ when it comes to multicast broadcast across the wired/wireless network segments.

Just to also mention that even though some may work, powerline (EOP) adapters are not officially supported by Sonos, as stated in the system requirements here: 

https://support.sonos.com/s/article/126

Probably because of the connection interference that those products may encounter from time to time as other products are powered off/on in the household.

Example janky BT router: BT HomeHub 3 compatibility | Sonos

OK.This is making more sense now. I know I said it’s been working well for several years but I used to have Shell broadband which I was running through a DrayTek Vigor 2762n router. Had no problems. I’ve recently moved to BT and am using their Hub. Would you suggest I go back to using the DrayTek? 

That question is perhaps difficult to answer with any degree of certainty, as something else may have changed on the network too @phoenixphil1967, different WiFi channels etc. However if you still have that router and you can ‘bridge’ the BT router, then there’s no harm in giving it a try.

Can’t rule out though that your issue might also be related to the EOP power-line adapters, as they are not officially supported, as mentioned earlier. 

How do things work for you when not using the EOP adapters, I’m just wondering if that might resolve your matter?