My apologies to those who feel that I have wasted their time and thanks to those who have tried to help.
I don't know how I managed to start two threads but this whole business well illustrates the confusion that I originally wrote about.
The 'technical' question was 'Why does exactly following the process that has worked perfectly well for other distros, using exactly the same hardware, fail when I try to install Debian the same way?'
The confusion was - and is - how do I separate my rather elementary and unsophisticated question from the mass of expert questions from sophisticated users so as to get an answer I can understand without irritating them and wasting their time?
That, I suppose, is a 'procedural' question, and I probably ought to have asked it separately.
I'm happy to drop this now, as it certainly hasn't worked for me, and look elsewhere for answers.
Josh.