Thanks, Choppin.
I tried assigning - MSB 112 LSB 0 (or blank) PC (53 for the GM VOICE DOO from the tutorial manual Tone List) MIDI channel 5 - to one of the controllers on the A500 but couldn't seem to get it to play the sound. Still doing something wrong.
Coincidentally, Dave Curtis has just put up a similar post on the FB page with good replies at https://www.facebook.com/groups/Casio.Px5s/permalink/1157121584413418/ - there are couple of things in there I will need to try but otherwise, will have to find a similar A500 forum with such great community support as for the PX5S.
Thanks, again