Thanks for your comment. Even though the FS documentation is wrong and confusing when you consider midi and FSchannels I know you are not going to change it.
Your argument that these channels are equivalent may be right a simple case such as your command interface but when you consider an organ they most definately are not the same or even equivalent. And it is very confusing to think they are.
The midi inputs for eplayOrgan arrive in midi channels whether from a keyboards or midi files. There is ONE midi channel for each input. Each midi input is processed and then sent on to the synth. The notes from EACH incoming midi channel are fed simultaneously to 32 FSchannels for processing by the synths. (eplayOrgan actually uses four synths - one for each division).
Also midi channels contain midi events in a specified defined format. The FSchannels are driven by software procedures in a completely different specified format.
Thus there is no way that midi channels are equivalent to FSchannels.
Now the good news - Now I have changed the channels from midi channels to FSchannels, fluid_synth_cc(synth, FSchan, 1,100) now works and produces tremolo using the sound font Tom successfully tested.
I have learnt quite a bit about sound fonts - especially how their modulation system works. I hope to learn even more by experimenting now I have a working system to play with. So thank you all for your help.