Hell! And thanks.
With your input added, I'll declare the Nektar unsuitable for my purpose, with thanks for saving me the trouble.
Meanwhile, I took a closer look at photos of the 61; the fronts of the keys look rather odd to me:

So, to get myself going with Hammond-ish playing, it'll probably be one of the Casios, or otherwise I'll put the whole idea to rest for a bit.
Here's where my experimentation led to so far (not the final setup, just experimental):

With on top the 'drawbars' and other controls 'programmed' in a cheap-but-nice-enough little MIDI fader controller.
Cheers and again thanks,
HZ