@Marcus - many thanks for your help.
I tried directing the output from fluidsynth to a file but it was too brief to reveal anything.
I also used 'aconnect -l' to determine whether it was the midi connection from my keyboard that was not being made correctly but everything was as expected;
Next I tried using aconnect to manually disconnect and re-make the midi connections with fluidsynth in both its 'not working' and 'working' states but again the outcome did not change. Removing the midi.autoconnect command-line switch and connecting using aconnect once fluidsynth had started also made no difference
This kind of made me think that the issue was not with the midi connection so I went back to looking at whether it could be the alsa audio connection.
I had received another [email-only] reply (thanks Alberto!) suggesting that I try setting my Pi to automatically create a login as root at boot time and to use the root .profile to run fluidsynth. I decided to give this a try and the first attempt failed again but I could at least see that fluidsynth was now running with an attached tty.
Purely by accident I ran 'sudo su' on another SSH session (and thus inadvertently started another instance of fluidsynth) and suddenly somehow fluidsynth started producing audio!!
To cut a long story short after more 'tinkering in the dark' I now have my system working as I need it to (albeit with a nasty root-login security hole - unimportant to me as this is going to be a stand-alone box).
So I now have :
1) a '/home/george/startfs' shell script which contains:
==========
/home/george/fluidsynth -s -i -o midi.autoconnect=1 -m alsa_seq -a alsa -g 1 /home/george/GeneralUser_GS_v1.471.sf2 &
==========
3) root's .profile contains:
==========
/home/george/startfs
sleep 2
killall fluidsynth
/home/george/startfs
Somewhat bizarrely fluidsynth only produces audio via alsa having been started once, killed and restarted, but this approach does now work reliably.
When I get chance I may investigate using jackd or a different audio connection method, but for now I have what I need - albeit a bit of a nasty hack!!
Thanks guys for the help.
Kind regards,
George
((Somerset, UK))