Do your speakers 'duck',  i.e. lower volume,  at the wake word?

Yes they do @John B 

This solved this issue for me: Tips & Tricks - Resolving random issues impacting Sonos devices... | Sonos Community

Followed by Disabling then Enabling the Sonos skill within the Sonos App.

Thanks @UKMedia it seems to be getting worse. I think my alexa in the kitchen has gone senile… she doesn’t even seem to know her own name when you stand directly in front of her… yet my lounge will pick it up.

She’ll also randomly kick into gear when I’m on conference calls and someone says something nothing like ALEXA but say Teddington… she’ll spring into action… 

Hi.

Did you reserve addresses for your devices?

@UKMedia no, but i have done this before and it didn’t solve it. The problem is two-fold:

1. Alexa rarely recognises that you are addressing her
2. When she does, after 3 or 4 attempts, she doesn’t action the ask… just goes quiet until you repeat, at which point it tends to work.