@Oldred 

I think I have fumbled around sufficiently to find a solution or workaround, possibly two although one seems preferable to the other. 
Because I have spent a lot of time trying various things without fully understanding the exact roles, nomenclature and relationships between them I could perhaps have got there more elegantly or sooner but the lack of explanatory documentation has been a surprise. It's as if clever people probably developed it and handed it to different, uninformed people who documented the bear minimum without any real understanding themselves. Sorry but that is how it has felt today and I should say I am a big fan of the Tapo products. I have tried to document what I did just in a se it helps someone else. Apologies if it is a little fuzzy but I spent quite a lot of time unclear on what was supposed to be happening :) 
 

Approach A was to turn off the Tapo automation that rang the H200 Hub's 'bell' and in effect recreate that same routine within the Alexa 'world'. I can then disable and enable Alexa routines via voice command. This did work in tests but was sluggish to re enable and once triggered it seemed to have an extra delay before it could be triggered again. It was as if Alexa routines have an inbuilt delay before activation. I couldn't find that documented anywhere in the time I had and so continued to search for a local solution using Tapo again. 
 

Approach B was to discard the Alexa level routine. I remembered that a well as sensors I had purchased a 'Tapo Smart button'. It was very unclear as to what this would/wouldn't control from the user guide. To jump to the end, I established 2 smart actions and applied them to the single tap and the double tap of the smart button. One action turned off the automation for the Tapo sensor'sntriggering of the Hub Bell, while the other turned it back on. So I now have a physical switch that I can place near the back door that will stop it ringing with a click and reinstate it with 2. Better still, creating and naming that switch command was automatically recognised as a 'scene' by Alexa. So I not only have the manual switch but also Alexa commands which is what I was pursuing. 
 

It's not obvious to me why the switch 'what ifs' are viable to Alexa while the 'smart action' what ifs aren't. Even less clear, is how a user is expected to discover that. The main thing is that there was a solution and it seems more responsive being driven by the local hub rather than the cloud. I am also left thinking that I don't think there was a solution without the smart button which allowed me to access these scenes but I don't need to ever use the physical button. I will install it nevertheless for visitors and Alexa averse friends and relatives :) 

the specific may not be so relevant to others but perhaps the mechanisms are.