Could you humour me and see if temporarily disconnecting your Sonos system makes any difference.
Thanks for your help. An offer came up for Sky that worked out cheaper than BT.
Now with them with their Sky Q router and it's fixed the issue. Must have been the BT Hub which is what I suspected.
Interestingly, I now get 29mb down whereas I only got 19mb max (when working) on BT.
It's rock solid too. Wifi range is a lot poorer on the Sky Router however.