So what is the devices hardware version in the ‘Settings/System/About My System’ (maybe take a photo and post that here too).

Have you tried moving the speaker nearer to the router to see if that gets it working for you. If it does, then it’s probably out of range for your wireless network, or perhaps something nearby (or in-between) is interfering with its wireless connection.  

All these are Play 1 but the kitchen unit is named Sonos One. It’s the kitchen that won’t come on with the rest of the group. However, it will work when I select play all speakers. It is the second closest to the router. It is annoying that it does not come on with the alarm even though it is part of the group. Is it significant that it is a different named unit to the others? i.e Sonos One instead of Play 1?

Well the hardware version shows the speaker as a Sonos One (gen 1), but it should look different to your other players - Amongst other things, it should have ‘touch’ controls whereas the Play:1 has actual buttons for play/pause/volume control. That said, it would not likely be the cause of your alarm issue. It’s more likely something is interfering with its connection to your router. Maybe try this:

Set your router’s 2.4Ghz WiFi band to use either channel 1, 6 or 11. Try 11 to begin with and set the channel width to 20MHz only and see if that fixes your issue.

Power the One off for 30 seconds and reboot it. 

I didn’t realise I still had a first generation unit. That would explain it. Because all the units are wall mounted high up in the corners, I had not noticed the controls being different. I will change it. Thanks for all your help.

The Sonos One (gen1) is newer than the Play:1 speakers - the latest model is Sonos One (gen2). There is no need to change any of them, all should play/work happily together in groups. See the suggestions of rebooting the speaker and my suggestion for changes to your router 2.4Ghz WiFi channel and channel-width.