@RG58 

If you are only buying Deco mesh for WiFi coverage, configure them to run in Access Point mode. It'll keep your ISP provided router in play and you'll avoid a lot of headaches.

@Alexandre. Thank you very much for the info.  It sounds ideal. I have tried all sorts of wi-fi extenders and old routers as access points in the past, and although I can get the coverage, it is in no way seemless on handover.  Hence why I am purchasing the Deco system.  

@RG58 

See below the corresponding setting, in smartphone Deco app. After you configured first Deco, which will be your Main Deco, change that setting as shown and add other Decos to your mesh afterwards.

@Alexandre. Thank you.  I appreciate your help.  Have you an opinion on leaving the original router's wifi transmitting?  I only ask in case my Macbook needs seperate 2.4Ghz and 5Ghz bands as it does with Plusnet's Hub 1 router.  I assume that although the Deco does not allow for seperating the wifi bands, I would be able to use a wifi analyser app to see what channells it is transmitting on so I can change the router channels to stop any crossover on 2.4Ghz by only using channels 1, 6 or 11?

@RG58 

Deco does allow for separation of WiFi bands. It runs two WiFi networks: Main and Guest, you can use it to separate 5GHz from 2.4GHz on these two networks. This is what I did in my house, and turned off WiFi on ISP router as I no longer needed it.

My Deco app WiFi configuration settings screen:

@Alexandre. Oh good.  The inability to seperate the bands was just something I read online earlier today. Glad it's wrong.  Thanks for your help and I look forward to getting it and setting it all up next week.  Sounds straighforward now.