As predicted in my last post, this hasn't been going according to plan.
I was unable to upgrade the previous Webserver SD Card to Buster because I got multiple errors relating to IP Tables. I suspect this is because the Webserver Pi has a non-standard firewall, which is all part of nodogsplash; the captive portal software.
So reluctantly I eventually gave up with that and started to build a new SD Card from scratch. I installed Buster Lite OK and was able to install the various Raspbian packages needed to make the webserver work. The problem came with installing nodogsplash itself. There is no Raspbian package for this and it has to be cloned from GitHub, compiled and installed in situ. It was the last step that broke. There is a known issue (flagged 7 days ago) in the makefile that causes the installation to fail.
The problem is that the nodogsplash package has recently been split into two parts and I need a lot more understanding of what the second part does before I risk applying some kind of work round (assuming it is valid to do so). The good news is that nodogsplash is obviously in active development (the last commit was 18 days ago), so I would rather wait a while to see what happens with the open makefile issue, especially since we will have to wait for the new broadband to be installed at WMT before we can deploy it.
So I'll put this on the back burner for a few days at least.