@StuartMoss 

Do you have Guest network enabled? By default, it is not password protected. If Guest network is on and without password, either turn it off or add password to it.

If that happens again, next time before blocking it check if unknown device connected to Main or to Guest network. You should change the password of WiFi network that device connected to, and don't give new password to anyone.

Finally, is your birthday coming soon? Perhaps, one of your family members preparing a present for you.

@Alexandre. I do not have a guest network enabled. I have a wifi password set up from the moment of installation of the Deco system which was less than a week ago.

Is the message that I recieved the message "Galaxy-S8 joined the Deco network" confrimation that somebody nearby has actually accessed my network? 

This is very worrying. 

@StuartMoss 

This message says that someone who knows your WiFi network password successfully connected their Samsung Galaxy S8 device to your WiFi network. You should have seen this device as active in Deco app under Network Devices, and to which Deco it were connected. Before you blacklisted it.

If that someone uses device such as iPhone instead, which can produce MAC addresses dynamically, blacklist won't help.

If you are concerned someone outside your household got hold of your WiFi network password, you should change it ASAP and don't share it with anyone, including people from your household.

@Alexandre. Thanks. I am slightly amused and slightly embarrassed!

I just had a lightbulb moment. I rang my brother just now. I asked if he had a Galaxy S8 phone. "Yes" Did you drive past our house today? "Yes, about 11:30"

I said it was actually 11:52 and I know that because my network told me so. "Oh, my phone must have connected automatically" Not any more I said - I've blocked you!

He has visited us many times before lockdown and has legitimately used our wifi on those occaisions, his phone obviously retaining the ssid and password.

So, mystery solved. 

Thanks again for replying to my original post.

@StuartMoss 

"New Device Detected" is one of overlooked but very useful TP-Link Deco features. While this time it were false alarm, if ever someone connects to your WiFi network without your permission, you'll know about it right away and can deal with it swiftly. 

Which is really important nowadays, because every device connected to your WiFi network appears on the Internet as located at your house.