I configured he HS103s as described in the support video by connecting my iPad to to each individual HS103's WiFi signal.

After doing this, the HS103 remained invisible to my main WiFi router. The only configuartion change I managed to make at this stage was to give then user-friendly names.
 

To document the actual steps I took in trying to configure a smart plug, I reset everything, including deleting then reloading the KASA app, and started to reconfigure the switches again. At the second or third try, I found out where I had gone wrong. Although the instructions seem clear as to whether the smart plug should be talking to my iPad or the house router, I find that the instructions are not so clear on whether the iPad should be connected to the plug's or the house WiFi. When I had moved the plug to the house WiFi, I didn't do the same with my iPad. So problem solved. My plugs are now doing what I need of them. 

So thanks for your intervention, without which I probably would just have put the plugs on a shelf somewhere and forgotten them. While the instructions could have been a bit more comprehensive, I lay the fault mainly on my 85-year-old brain.